Developing the parallel ideas a bit further, there are some time savers that you can apply to your initial proposal.

First, CLK and ACK don't need to return to their original state for the next byte. You already know all you need to know from one edge. So for one byte, CLK goes H to L, and the next byte CLK goes from L to H. Same thing for ACK. (Anybody remember Laplink?)

Second, CLK can be the data strobe from A->B, with ACK being the acknowlege. When B is talking, ACK is the data strobe from B->A and CLK is the acknowledge.

Who wins a collision? BUSY indicates that A wants to talk. When BUSY is asserted, B will listen, or if already talking will quit ASAP. After asserting BUSY, A will wait for a specified period before transmitting, enough time for B to clear the bus if it was trying to talk.

A knows that B is talking if A sees transitions on ACK when it hasn't sent a byte.