No laws against it. At least in the USA.
i think it is as long as you're not having s*x with both of them.
If the couple has not become exclusive - both the parties can move around with other potentials. If, however, the couple is exclusive - No.
If you are just going out having fun and there is no commitment then I don't see a problem.
not if ur lying to either one of them about it. but u should expect to be called some harsh names when they find out. and u also have to think "is it ok for one of them to date another girl at the same time?" if not, then u shouldnt do it either.
i see no problem, rock on!!
Yes, and don't brag about it to the other. To be open and honest, you can say to one, not tonight I have another date.