Sounds like a miscalibrated Talon. Look on page 7
here, and try calibrating the Talon - there's a good chance that this'll fix it.
As far as using a Victor SP, it's generally not recommended to use different motor controllers on the same gearbox. Different controllers have different response curves, so you'll probably end up with one motor doing more work than another. It's not necessarily unsafe, per se, just not recommended.