It takes a couple of minutes (if that) to set up an account with TFL.

Any erroneous charges (such as incomplete journey charges) can generally be resolved on-line with no need to speak to anyone at TFL.

I can't see anything on the TFL website about not having multiple cards on the same device but I don't think it's a good idea simply due to the ease of making a mistake and over paying as well as the hassle factor of switching between cards on entry and exit and delaying other people!