Hello
I have an inbound interface, let's call it IN, used by my clients (different MACs)
I have an outbound interface (OUT1) which is bridged to IN. Everything is cool.
Now I am adding a new interface, OUT2, and I want only 1 client to be bridged to OUT2, the others stay in OUT1
So,
Traffic from 00:00:00:00:00:01 in IN interface bridged to OUT2
Traffic from all other MACs in IN interface bridged to OUT1
Is it possible to do this with routerOS ?

Yes, but it needs some fiddling with /interface bridge filter rules. When client 00:00:00:00:00:01 sends any frame, that frame must not be forwarded out via OUT1; when other clients send any frame, these must not be forwarded out via OUT2. Similarly, eventual frames for 00:00:00:00:00:01 coming in via OUT1 must be dropped, and frames for other dst-mac than 00:00:00:00:00:01 coming in via OUT2 must be dropped.