How to filter multiple subcategories from one group vs another group?

Hi, a question with the Ajax Filter..

I have two groups where each of that boxes contains Sub-Categories of different Categories:

Main Categories: A, B
Sub-Categories group 1: a1
Sub-Categories group 2: b1, b2, b3

I need the filter “a1” active all the time, and filter in conjunction with “Group 2“; but only works selecting one of “b“s subcategories at time.

I can’t filter with two or the three subcategories at same time when the “a1” active.

How can I do that? I can’t achieve this. I’ve the pro version.

Thanks