Depends on the breakers. Some are literally two single breakers with a handle tie. Those you can do exactly that. Actually, other than the nuisance factor of one tripping and taking out the other one, there's nothing that says you can't put separate circuits on each leg of a two pole or tied breaker.

firstof all, when you said for 30 amps total, you are wrong. It is 15 amps 240 volts. i would reccommend just buying a new 15 a breaker and a filler plate.

" i would reccommend just buying a new 15 a breaker and a filler plate."

Not really worth the trouble.

Use one side of the two pole breaker and leave the other unconnected.

If you need another 15 A circuit, use the other side of the two pole.

If it is annoying having BOTH circuits turn off when EITHER trips, purchase two single pole breakers at that point.

It is likely that both sides will not turn off when tripped, use them both for separate circuits.

Steveomc, it has become clear to me that you shouldn't be giving advice on this forum. You clearly don't have the requisite experience/knowledge.

A two pole breaker is designed so that both poles open on a trip.

Changing to single breakers could avoid confusion and wasted time in the future.

what do you think the ''common internal trip'' is for? Read a breaker. i dont mean to be rude