Given that each radio only has one transmitter it is not possible for two users to transmit at the same time. Transmit priority is given on a first one wins basis.
Each radio has a number of specific number antennas that the radio can support simultaneously. This is the number of Spectral Capture Units (SCUs) in the radio. There is one SCU in the FLEX-6300, FLEX-6400 and FLEX-6500 and two in the FLEX-6600 and FLEX-6700. If you have two operators on a single-SCU radio, they would each need to use the same antenna, but could use different bands (provided the antenna works on both bands). For the dual-SCU radios, each operator can be on a different antenna — or they could share antennas. For example, on a FLEX-6600 or FLEX-6700, operator one could be listening on 20m (beam on ANT1) and transmitting on 40m (dipole), while operator two is doing the opposite (listening on 40m and transmitting on 20m).
If you have a single antenna that is wide-banded, for example a Tennadyne T10, and say a beverage, both operators could have the T10 as their transmit and receive antenna (single SCU) or one/both could transmit on the T10 and listen on the beverage.