First of all, since it is a utility model and the patent right is not strong, it is best to make two patent search reports at the State Intellectual Property Office to see if there is any existing technology that affects novelty and creativity. If Yes, then A does not need to invalidate B, because B will also invalidate A. It is better for both parties to lose than to win, and the two utility models are cross-licensed. If A feels that its patent rights are stronger, A may consider invalidating B's patent.
B can either invalidate A’s patent, or pay some money to keep B’s patent.