Applying for Post Doc position with already one in hand
Typically different institutes, even within the same organization (e.g. the Max-Planck or Fraunhofer group), don't formally exchange information on applicants to their positions, so you don't need to be afraid that they won't consider your application just because you're in the process of getting hired somewhere else. However, you shouldn't underestimate informal information exchange - if you're in a small field, the PIs who want to employ you may well talk with others about that post-doc they are going to hire soon.

Moreover, if you've already explicitly agreed to an offer at one place, and they are preparing the contract, it perceived as very impolite to back out of that. Legally, it's still possible to do, and if it makes a really big difference for you, then you should probably do it, but be prepared that it will annoy the person you agreed to working for.