Great point. Allow me to expand....
I've drilled in sim-houses, done threat on threat etc....I'm no instructor but here's what I've gleaned from getting beaned during these exact scenario's:
It's no fun trying to run and gun to cover or another stashed firearm, esp when said stashed firearm is in, or on the way, to an occupied room. You are now running into harms way (exposed) instead of evacutating or seeking cover- which is priority one. If you have an out, take it. Every time.
If you cannot, and, must barricade:
It is way eaiser, safer and infinitley more efficient to carry one firearm and additional/back-up mags/ammo for same than blasting your way from room to room trying to get back-up firearms.