Sounds like you are not engaging the mid PTO before engaging the rear PTO.
The rear PTO, I believe, engages through a clutch (admit the 790 may be different). The Mid PTO is just a mechanical connect with the rear PTO.
At least that is the situation with my 4300. i.e. cannot have the rear PTO engaged and then try to engage the mid PTO. Make sense?
Or does this not fit with what you are having problems with? ?