> Is it likely that the other cruciate will go
I think that any dog who has had an AC rupture is more at risk of the other going. Boxers, Rotts and other powerful, muscular dogs put an enormous torque on the stifle joints when running and, especially turning at speed.
The fact that one has gone may point to a weakness. I was warned by the vet surgeon ,who did my bitches op, that this maybe the case.My bitch was wrapped in cotton wool after her op as I was never totally sure of her soundness.
Swimming is terrific exercise for them in their recovery. I found my bitch was pretty wacked out after her sessions and slept like a log, it really takes the fizz out of them.
I do hope your outcome is good.