my second donation would be today Monday, but if i skip and come on Tuesday would i then be able to donate on Thursday?

Depends on your first donation. Donation days begin on each Sunday and ends on each Saturday. So if your first donation day was Saturday and then you wanted to skip Monday and come in Tuesday, then your days will then become Saturdays and Tuesdays. You can push days forward but never backwards.

Depends on when your first donation was you're only allowed 2 donations in a 7 day period