I am presuming that you want the two machines handling this port at different times. [For example one during the day and the other at night.] Even with schedules enabled the router may not have this ability. After playing with the virtual server, setting up a few nicely spaced schedules and in choosing live machines to forward the same port to, I received your exact error.
This possible workaround just occurred to me, haven't tried it but give it a shot:
1. Keep your same WAN side public port;
2. But, on the LAN side make the private ports of the machines different. For example, port 84 the first machine, and port 8080 for the second period.
Now, as long as the schedules don't conflict, things may work .
Patrick