There seem to be many different scarf
The garage I'm building calls for ~23' long rafters (8x8) supported by purlin plates and eve plates. I'd like to use two timbers for one rafter and use a scarf joint over the purlin plate. Is there a certain scarf joint that would work best in this scenario?
Sometimes you need to use a scarf joint that works for how you're going to erect the frame.
If you're going to assemble the two halves of the rafter on the ground and then fly them up there as one by a crane.
Or are you going to fly one half up and then the second half.
There shouldn't be a lot of end to end tension in the rafter pair, so a scarf that could work would be a bridle scarf joint show here: https://timberframehq.com/timber-frame-bridle-scarf/
But that should be checked by a qualified, and experienced timber frame engineer, to be sure.