So your idea is to just make the toothed length 3.5mm more wide…
Not sure it’s THE solution. The belts won’t be wedged and “Forced” rubbing against each-other, that’s for sure.
The construction will always cause some movement of the belt running either upward or downward.
You can’t exactly position the motor pulley perfectly aligned with the rod, that’s the biggest factor and will possibly cause it to make contact anyway.
Also going from 15.5mm (orginal flange to flange) to 24mm (flange to flange) in length will cause the shim side pointing to the bearing to rub against the bearing holders.
(I had grinded a few mm’s from the bearing holders to prevent this, mine flange to flange is a total of 20mm)
So It’s pretty essential to resize the total thickness where you thread in the grub screw yours is 6mm thick. Grub screw is 4mm long so take ~2mm or something.
(the original thickness that’s threaded is only 1.75mm)
IMHO it’s better to add a shim in between so the belts can’t make contact AT ALL!
So while you’re busy producing some fancy custom pulley I would go a step further than only increasing the length with 3.5mm…
That’s my opinion, after experimenting with them myself for a annoyingly long time.
My Setup for over 10 months!
I did this with 2 single belt pulleys, which I trimmed off and have the grub-screw hole positioned in-between 2 tooth.
Works great so far…
The biggest issue I have experienced are that the grub screws are hardly ideal this way, they need to be flattened/shortened a bit so they are completely countersunk with the bottom of the tooth, that makes them really tricky to handle and not ruin them entirely!
But it runs amazingly and unexpectedly smooth like you have never seen before!