I've got a multiwire branch circuit that I believe is wired improperly. From what I understand these circuits can be dangerous if wired with both hot conductors on the same pole (same side of the panel). Am I correct to say that they should be wired on opposite sides of the panel? Can I take one of the hots and extend it to the proper side of the box with a pigtail, or do I actually have to pull a longer length of wire to get it there (very difficult at this stage in the game). This is a 100 amp service that supplies a 425 square foot apartment in the upper level of a house so the panel is not crowded by any means.

Next to each other is best and on a 2 pole 20 amp(if #12)breaker.
They need to be on Opposite phases.
240 volt circuits uses each phase.
If both are on the same phase neutral current can go as high as 40 amps and heat/burn up.
You can verify you are on opposite phases by measuring the 240 volts if wired correctly.
If you were measuring across the same phase, the voltage would be zero.

Don't circuits wired with 14 gauge wire need to be on a 15A breaker max?

Yes, you didn't say, and I said 20 Amps(if #12).
It would be 15 amp 2 pole breaker for #14.

Sorry, missed the parentheses. This makes sense to me now. Thanks for your help!

Good Deal. Anytime

Any chance you have a Federal Pacific panel?

Square D QO.