That could be joint enterprise, which would not apply in this case.
Why not, both are conspiring to protect each other knowing one of them committed the act?
Because it requires forethought. Is it reasonable to assume that when you get into a car with someone, that they are going to suddenly decide to ram someone off the road? No, of course not, hence joint enterprise would not be valid.
Would it be reasonable to assume that if you go out with a group of lads and one of them has a baseball bat but doesn't particularly follow the sport, that ABH/GBH could occur? Potentially, yes.