If you build a double on the 45-120 or 50-120, all you end up with is a very expensive oddball that no one wants. The savings on brass will be more than eaten up by the added cost of building the gun.

As for brass, Midway will sell you a box of 20 470 NE brass for $32, or 500 NE for $34 (on sale at this time). That's not very expensive when you are talking about feeding a rifle that cost $10K+ at least to build.