If you get there early (rope drop) I would get a FP+ for TT and ride standby for Soarin. But if you plan on getting there later I would do a FP+ for Soarin and ride Single rider for TT.
Exactly what we do. Yes they split you up, but it isn't until you board the vehicle.
One tip for single rider Test Track: In the single rider line, you don't get to do the extensive design that the FP/Standby riders do. After you exit the ride (or if you go in via the gift shop and thru the showroom) there are three design stations where you can design a car. This design is linked to your MB/ticket so you can use it next time.