I think you could definitely benefit from using two bodies, whether or not one is a crop body. The reason is simple: no lens switching. If you're shooting with a long lens on one body, and something happens right in front of you, just grab the second body with a wider lens and fire away (Black Rapid strap is perfect for this).
I'm not a sports photographer, but I imagine this is a standard technique for many sports and other types of shooting. Heck, I even use this technique at weddings. In your case, you would benefit from having the long lens on the 7D (1.6x more reach) and the wider lens on the 5D3 (more wide angle range).
Of course, you could sell your 7D and probably put a big dent in the cost of some nice L glass. If you're lacking a lens or two that you think you would really benefit from, then this might be the way to go.
It all depends on your shooting style and your lens kit, but everything else aside, shooting with two bodies is more flexible than one body.