I got the Modfactor for the TremoloPan Too! although I'm still playing around with it I can give you a an answer in principle??
The second LFO can be set to envelope follower. If you set it to only effect the amplitude (depth) I think it will do what you are dreaming up. You'll have to mess with the depth amount and sensitivity to get it right but I'm sure it's possible 🙂
