If you don't have small children and can split up, just ride Soarin first and then do Test Track in the single rider line. My husband and I rode in the single rider line at all times of the day last June without waiting in line for more than 15 minutes. Since the cars you "design" only work about half the time, we decided single rider is the only way we'll do Test Track.
Good luck! In June, the standby lines will be LONG early in the day. Whichever ride you do second will probably have a 45 minute + line by the time you get to it. Last June we were some of the first people through the turnstiles at rope drop on our Epcot day. We sent the men to get Soarin' FPs right at rope drop while we took the kiddos to the Character Spot. By the time we all met back up at Test Track - literally at 9:05, the standby line was 50 minutes posted and 35 minutes in reality.