I'm not aware of any way to choke calls to a ring group, that still has idle members.
I suppose you could send the call out on a loop trunk that is limited to two calls, then direct the incoming trunk to the ring group. There would have to be a second route in the outbound rules that sent the "overflow" to the queue. the problem is, it's an outbound rule and you want the remaining calls to go to an internal route. Using the loop trunks will also "eat" up two licences per call, so not really a viable solution.