Just some notes...
There are many different manufacturers of circuit breakers and many of the breakers are inter-changeable. Square-D makes QO, QOB, QOU, etc., and Home-line breakers, The home-line style is the most common (in this area) for residential use. These types of breakers will fit in (most) panels manufactured by Murray, Cutler Hammer, GE, Siemens, etc.
In the old days, they used to use SE cable (or 10/2, 6/2, etc., romex) to wire stoves, dryers, etc. A black, a white and a ground. For a 220V device (water heater, dryer, etc.) the loads are the same on each leg and cancel each other out on the neutral. Now for new installations you need to wire for a 4 wire connection, but that's getting off the point.
There have been some good suggestions on how to get around the $100.00+ Eaton Cutler-Hammer GCFI problem. 50A regular breaker to a 50A outlet. 60A sub-panel (about $15 at HD) wired with a 50A (cheaper) GFCI breaker. Basically a 50A extension cord with a GFCI protected sub-panel in between.
If youre going to have 2 separate circuits share the same neutral, they should be connected to a 220V breaker. (required in some areas) This way you dont have current on the shared neutral with only one of the circuits off. It also eliminates the risk of putting both circuits on the same leg and adding the current on the neutral (possibly overloading).