How to cut-extrude mixed parts in an assembly

Hello everyone.

I've drawn two parts as you can see in the image below and I don't know how to properly proceed in an specific feature.

I'd like the #1 part had the #2 part's shape so they can mate this way. In fact I don't know how I can remove this mass on #1 part..

Do I do this in the assembly or in #1 part's sketch?

I hope I was clear.

Thanks in advance.