Is it morally incorrect to hook up with 2 people at the same time having told one of them that they are exclusively the only one they're hooking up w?

Doesn't matter if it's morally incorrect. What matters is that you're exposing another person (the one you're supposed to be with exclusively) to possible STDs by having intercourse with other people. The shitty part is not telling them that they're taking that risk with their body.