I have a sub panel in my attached garage. It has a small heat pump/air
conditioner on it for a bonus room (I believe 50 amp) and a 50 amp
welder plug that rarely gets used.

I built a detached garage /shop with a 100 amp panel. Can I run my sub
panel in my detached garage to the sub panel in my attached garage?

I have the capability to attach it to the main panel but it would be
alot of work and would have to move 2 circuits.