So sorry for the lengthy delay in responding, I've been on the move and not checking in as often as I'd like to.

On versions of Solaris where I've tried this (2.6 and 8 only) the Solaris package database could not handle multiple operations at once. If I attempted to install two packages at the same time, one would wait for the other to finish before starting.
It's purely anecdotal evidence and only observed on older Solaris versions but I would be surprised if it didn't do it on recent Solaris instances also.

The easiest way to find out and see for yourself is to start a package install or remove that pauses for input along the way, then start another operation and see what it does.