"I'm pretty sure there was a away you can change the tempo of an individual track channel."
MMmm, not that i know of...
My suggestion:
Use these 2 Actions, made by me2beats, for changing to half time or to double time:
Script: me2beats_Set items length to 0.5 (stretch items).lua
Script: me2beats_Set items length to 2 (stretch items).lua
For availability of these 2 Actions in your Reaper Action list, you need to:
a) have the free Reapack installed:
https://reapack.com/
b) me2beats repository URL added in Reapack:
https://github.com/me2beats/reapack/...ster/index.xml