I can't quite picture what you are saying or doing. The breakers are in the panel and are fed by a main or are you creating a sub panel with 2 breakers?

What size is wire feeding the panel. This will control maximum size of main breaker. U can not use a main larger then rating for panel box.

Chuck

Can you use more words to explain what your doing, what you have, and why your doing it?

Based on the small amount of information given, the answer is #4 copper wire is needed to feed the panel.

IT's a sub panel rated at 70 amps with is a two 15 amp breakers only.The question is what size breaker can I should I use to supply the panel is a 40 or 50 amp OK .sorry for the confusion.

I will assume this is a two circuit panel, with space for only two single pole breakers.

You can you a two pole 15 amp breaker at the Main panel and use #14-3 plus ground cable to feed this panel.