Does a double thread, starting 180 degrees apart, make it easier for the user to mesh the threads and allow a greater travel distance with fewer revolutions?

The ISO Metric thread M90x6 might suit your needs. It moves 6mm per revolution, so would do 9mm in 1.5 turns. Give a good chamfer to the top edge of the container and the bottom edge of the cap and it should be user friendly.

Double start threadformula

Divide 8.5 by 1.5 to get the pitch for one revolution. 5.6666 is close to the standard M90x6 thread.  M90x6 thread will move the 8.5mm in 340 degrees.

Of course the travel distance per revolution doubles relative to a single-start thread that otherwise has the same specifications.

Double start threadtap

Divide 8.5 by 1.5 to get the pitch for one revolution. 5.6666 is close to the standard M90x6 thread.  M90x6 thread will move the 8.5mm in 340 degrees.

My reading of the OP's problem statement leads me to believe that a standard screw thread for machined components is not the solution for their Design Intent.

The typical thread specifications we are used to are built around the thread form itself, and if you want to turn any given size into a 2-start configuration, you will have to up the helix angle. Now of course your pitch will be doubled, and you might not want that, in which case you would have to select a smaller size in order to compensate. Is that what you mean?

Double start threadpitch

Does a double thread, starting 180 degrees apart, make it easier for the user to mesh the threads and allow a greater travel distance with fewer revolutions?

P.S.  I would not use Fusion 360 for this helical form features - I would use something like Autodesk Inventor Professional.

Singlestartanddouble start thread

Unless I miss-understand the Design Intent I would model a plastic bottle thread (in Autodesk Inventor Professional) rather than a 60° v-profile.

Actually, it does need a smaller cross section, otherwise there would be no room for the second start.  Is there a reason for not using the trapezoid form?

You can create custom threads by following the instructions here.  For a two start thread, use the TR90x12 or TR90x18 thread as the template and change the MajorDiameter, PitchDiameter, and MinorDiameter to match the TR90x6 threads.  The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing.

Double start threadprogramming

Unless I miss-understand the Design Intent I would model a plastic bottle thread (in Autodesk Inventor Professional) rather than a 60° v-profile.

You can create custom threads by following the instructions here.  For a two start thread, use the TR90x12 or TR90x18 thread as the template and change the MajorDiameter, PitchDiameter, and MinorDiameter to match the TR90x6 threads.  The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing.

Multistart thread

A double-start thread gives the user more opportunity to start the thread, but the EASE of starting it depends more on the thread profile, size, fit, and the end-treatment of the hole and screw/bolt.

The typical thread specifications we are used to are built around the thread form itself, and if you want to turn any given size into a 2-start configuration, you will have to up the helix angle. Now of course your pitch will be doubled, and you might not want that, in which case you would have to select a smaller size in order to compensate. Is that what you mean?

You could certainly choose to do things that way, but I think the typical implementation just increases the helical angle such that the the threads on the first helix is large enough to fit the threads of the second helix in between them.

Multistart threadPDF

Of course the travel distance per revolution doubles relative to a single-start thread that otherwise has the same specifications.

End Mill, Milling Cutter, Drill Bit, Carbide End Mill, CNC Tool Holder, Collet, End Mill Cutter, Twist Drill Bit, Carbide Inserts, Carbide Reaner

Yeah, I'm pretty sure that the Snapple glass bottle (or similar) 3-start thread (the cap itself is not threaded, but makes use of three crimps to serve the same purpose) is not an ANSI , ISO or JIS (etc.) standard. Glass bottle manufacturers have their own standards. They make up whatever they want, and typically they want something that can be formed in the bottle making process, which precludes expensive glass machining processes that would be required to make profiles fitting some of the more common standards we are used to.

Double start threadchart

Actually, it does need a smaller cross section, otherwise there would be no room for the second start.  Is there a reason for not using the trapezoid form?

Yeah, I'm pretty sure that the Snapple glass bottle (or similar) 3-start thread (the cap itself is not threaded, but makes use of three crimps to serve the same purpose) is not an ANSI , ISO or JIS (etc.) standard. Glass bottle manufacturers have their own standards. They make up whatever they want, and typically they want something that can be formed in the bottle making process, which precludes expensive glass machining processes that would be required to make profiles fitting some of the more common standards we are used to.

The ISO Metric thread M90x6 might suit your needs. It moves 6mm per revolution, so would do 9mm in 1.5 turns. Give a good chamfer to the top edge of the container and the bottom edge of the cap and it should be user friendly.

You could certainly choose to do things that way, but I think the typical implementation just increases the helical angle such that the the threads on the first helix is large enough to fit the threads of the second helix in between them.

P.S.  I would not use Fusion 360 for this helical form features - I would use something like Autodesk Inventor Professional.

A double-start thread gives the user more opportunity to start the thread, but the EASE of starting it depends more on the thread profile, size, fit, and the end-treatment of the hole and screw/bolt.

My reading of the OP's problem statement leads me to believe that a standard screw thread for machined components is not the solution for their Design Intent.