Can I set up 2 HomePod mini speakers paired to my computer while using the computer speakers at the same time

It may depend on the computer. In some cases you can AirPlay to multiple targets, and still use the local speakers. See Use AirPlay to stream audio - Apple Support. I think Mac has better support for this than Windows.

tt2

HomePods are great, but there is a lag with AirPlay that makes them unsuitable for gaming or live music creation. Having traditional speakers will help if you do either of those.

tt2