When hero A counters an attack, subsequently hero B is able to use his/her ultimate skill, hero A becomes unable to chain ultimates with hero B in that same turn.
Thanks for the report. I am not sure what you mean by "same turn", because it should be enemies turn if a hero counters, and players turn when you activate B's ultimate skill. We will try to reproduce if possible but so far we haven't seen any ultimate skill chaining issues.