Oops I misread your post and thought you were using the two breakers of the
same value on different legs in place of a double breaker. That would
indeed be electrically the same though not a good idea for reasons cited.

As for the question you really did ask a 30 A two pole breaker would pass
30 A on each leg, not 30 total. In 240 or 208 V usage 30 A would be going
out one line and back the other so still just 30 total as it's across two
hots. Naturally 30A breakers would not offer proper protection on a 15 A
circuit.

And on the 2nd question two individual 15 A breakers would not suffice to
feed a 30A recepticle.