I can’t see that there is any prohibition against combining two HVAC condensate drains into one. I see it occasionally and thinking it to be poor practice, I usually say this about it:
“The condensate drains for both systems are jointed together. A clog in one line with affect both units and we therefore recommend, as is customary, two independent condensate drains. Service by a licensed HVAC contractor is recommended.”
For clarification, two systems. Usually in the attic. The drains join together, usually in the attic, and one pipe discharges to the outside or waste receptor.