Basically, yes, there are electronic considerations in doing this. (BTW, I'm not referring to the TF or any specific device by any manufacturer.) Can it be done? Yes, but there's a reason that they make line-mixers. Will it work? That depends on if you like the result. As goldglob suggests, impedence and phase cancellation are several considerations, as are signal strength, grounding considerations, and others. Then again, you may like what it does and I doubt that it'll seriously damage your pedal or amp.
I think a better question would be why do you want to do this? I'm not trying to suggest that it's a bad idea, but rather just curious what you're looking for, what the result is that you want to achieve. Maybe there are other ways to achieve it.
