Different hot circuits connected together
Hmm. If you have two hots connected and they are connected to the same buss bar in the service panel, then the result will be... 120V. You could end up with more amperage on the neutral or neutrals than the circuit breakers are rated for, though. A picture might really help, here. If the hot wires come from different buss bars, you'll have 240V on the wire because that's the total potential from one pole to the other on the transformer outside your place. The voltage won't hurt the wire, but it could cook connected equipment. Plus, you'll also have a direct short resulting in lots of amperage, so it should trip the breakers and nothing would work at all.

It sounds like if indeed you have hot wires from two different branch circuits connected together, they are on the same leg (same buss bar). The easiest way to see which one goes to which breaker is probably to undo the wire nut, flip breakers off and use an electrical tester to see which breaker cuts the power in which wire. If they're disconnected from one another, each wire should be energized at 120V.

While it is conceivable that an electrician made a mistake, I wouldn't hesitate to call in an electrician on that basis. Licensed electricians are professionals, by and large in both name and practice, who have had enough training and hands-on experience working as a journeyman for a licensed electrician to qualify for their professional license, plus the subsequent experience and continuing electrical education as a licensed professional. If you're worried about one particular electrician, call a different one.

If the one you think screwed up is a personal friend and you're worried about hurting their feelings or not getting invited to the next wedding, then call in the "replacement" when the original guy is really unlikely to just stop by, like during the middle of the day when both electricians are likely to be busy with business.  :-)