Hi Scott K,
You have a number of options, but you have to decide where you want to compromise...
* If you want two amps into a single mono cab - then look at the various head switchers on the market (products from Radial and Axess have already been suggested). They are fairly expensive because they have complicated circuitry that maintains a correct load on the amp not being used, whilst the mono cab is connected to the amp that IS being used. The strength is that all of the speakers in your mono cab will be used at all times. The compromise is that you cannot use BOTH heads at the same time.
* If you want two amps into a single stereo cab - then all you'll need are the speaker leads to connect them up. The compromise is that only half of the speakers in your cab will be used at any one time.
* If you want two amps into two mono cabs - then look at the dozens of AB and ABY pedals on the market. The pedal is placed between your guitar and the two amp inputs, and because such pedals work with guitar level signals, they are pretty inexpensive. It is worth paying more for transformer isolated designs however (e.g. check out the Dual from www.lehle.com ) as they will avoid humming loop problems. AB pedals will often only allow you to choose between amp A or amp B. ABY pedals also give you the option of running amp A and amp B at the same time.
An ABY pedal seems like the ideal scenario for you, but the compromise is that you will need those two mono cabs to cart around.
An interesting mix of compromises would be an ABY pedal with a single stereo cab. Half of the speakers would be used during A and B modes, and all of the speakers during Y mode. I have no doubt that a JCM800 mixed with a Mark IV would sound awesome! 8)
Big smiles,
Andy.