Dan,
What is the final topology that you have. What is the distance between these two stacks.
If you want to connect these two stacks for redundancy, I would suggest to configure the links as an ether-channel link. This will give you both redundancy and load-sharing with aggregated bandwidth.If distance is not so much then you can use the ethernet cable. If you have a larger distance, then you have to buy SFP GBICS for fiber connectivity. 3750-E comes with a TWIN-GIG converter to get installed in the 10GE port and give you the SFP connectivity.
Also, if you have backbone 3750 switch in the connectivity,it depends howz the connectivity is there in the topology and the kind of config that we have, the traffic will flow accordingly.
HTH,Please rate if it does.
-amit singh