NOTE: I AM NOT USING COMMUNITY BUILDER

I already know that i can specify that group A cannot send Messages to group B.

What i want wo achieve is following:

Group A can not send Messages to group B, until Member B1 from group B sends a message to Member A1 from group A. From now on, Member A1 should be able send an answer (only a answer, not a new message) to Member B1.

Another option would be that i define that group A cannot send messages actively, they can only answer to messages E.G. by disabling button "new message" for one of the 2 groups?

Is this possible?