I am running a 12 gauge 20 amp wire from a main box to a separate 70 amp box with 2-20 amp breakers to feed 2 separate circuits. With the one line coming in do I piggy tail the 2 brekers to the one 12 gauge wire coming in?

Not sure what you mean, but most assuredly as you say it it's a NO. One breaker, one circuit. If can you explain in better detail I may be able to clarify my answer.

Thank you for your answer. I thought I could do it but found out I can not. I have one 12-2 wire coming into the 70 amp box. I wanted to run 2 separate 20 amp circuits but now I know I can only run 1. I should have ran a 4 wire into box than I could

Why a subpanel for two 20 amp circuits, and the feeder is 20 amp?

I do not see a need for a 2 circuit subpanel, as you stated, run a 4 wire #12 with two 20 amp circuits and eliminate the subpanel.