While within their limits, amps are essentially voltage control devices, so the loads in parallel won't interact via an electrical path. There may still be mechanical interactions from shared air space or external proximity/boundary/acoustic loading effects but from an electrical standpoint, the two drivers don't "see" each other.
In a series configuration however, it is possible for power to be transferred between the drivers unless they're exactly the same. Any electrical or mechanical differences between the two drivers may cause energy to flow between the two, which can complicate their behavior compared to systems with all drivers in parallel. It's also possible that this interaction could couple with another interaction (say acoustic or mechanical interaction due to proximity or shared air space) in a way that leads to a feedback loop that causes unstable behavior.
In reality, the differences involved are probably too small for this to be a problem most of the time. However, I believe there are always opportunities for exceptions. Some manufacturers may be more consistent than others with regard to parameters that matter. Loading drivers into a tuned enclosure (especially one with higher pressures like a horn) could amplify certain problems. Running the drivers harder where non-linearity becomes a big factor is probably likely to accentuate such problems too.
But this is all really just theoretical speculation. I have no idea if any of these effects are really strong enough to cause serious problems.