A buddy and I did a couple multiple tank set-ups for a LFS
here in town. The first one was six 20gal longs plumbed into a single custom acrylic sump (about 10 or 12 gallons). First chamber in the sump was LR
rubble, second was the fuge w/ external skimmer, and third was the return pump. We used a QuietOne 4000 highhead as the return pump. Each overflow and each return line had a ball valve so that individual drain and refill rates could be controlled, as the tanks were stacked 2, 2, and 2 above the sump. As a result returns were almost fully open on the top two and shut down quit a bit on the bottom two. Getting everything working correctly was a big P.I.T.A. to start with, but once things got worked out it was no big deal. Make sure you put some type of strainer on both the returns and the overflows - the LFS
owner had issues with snails and fish in both.
The second one was two 55gal tanks plumbed into the same size custom actylic sump with the same set-up. They were also stacked, so again each line (overflow and return) had to have individual ball valves to control the flow. This one was nearly as difficult to adjust initially as the first one. Flow however was an issue in both of these - If I were to do it again, I would use an external return pump for this set-up; even a QuietOne 6000 didn't truly give enough return flow for this tank.
And I agree with Doug, I would drill not relie on external returns, especially not multiple external returns.