The UK procedure when cloud flying is not to enter from below if somebody else has climbed into it until there is at least 500 ft separation. Then each calls out altitude (amsl) periodically to maintain separation.

It seems to work. No collisions in cloud since this and other procedures were introduced.

FWIW, in my experience it is rare to be climbing in the same cloud as others anyway, but on the few occasions I have done it, it worked OK.

Chris N