simulate multichip designs

Functional simulation in ModelSim isn't related to a particular chip. It basically doesn't matter, if you have one or many DUT's in a testbench.

Typically, I use parts of other designs as auxilary components in a testbench. E. g. the peer of a communication channel is represented by the respective HDL module. I can simulate interesting aspects as effect of a clock frequency offset or transmission delay and delay skew.

The wiring of multiple DUT's or auxilary modules has to be done manually of course.