What I’ve found, is that each ANT+ dongle controls one and only one Zwift session. You can have a single bike + trainer + sensors controlling multiple Zwift sessions, but they must have their own ANT+ dongle mapped to them in order for that to work.
In other words, if you have two machines, each with their own ANT+ adapter, your 1 bike and 1 trainer can simultaneously control 2 separate Zwift sessions at the same time, one each running on each PC.
You can also run 2 Zwift sessions on the same PC, if you plug both ANT+ adapters into the same machine, and pair each Zwift session to your bike+trainer. This lets you run two simultaneous Zwift profiles at the same time on the same machine, tied back to your 1 bike and 1 trainer.
Does that make sense?