I think that if I make each batch with about 2-3 gallons of milk, I will be ok and then press them together. Cheddar does use slightly more pressure though but that won't be too bad. However cheddar usually ages much longer almost twice as long as jack. I guess if I make a dry version rather than a fresher version of jack then it might be ok. I think that I will add color to the cheddar so that I can see the difference in the final product.
I just have to wait for an international shipment of my cultures, and rennet. Might take six weeks.