I've never had much luck getting a 450 BPE and a 500 BPE to regulate with blackpowder, tried a lot of loads.
As soon as I changed over to a NFB load, they both regulated perfectly.

I just had a look through Graeme's books, I can't find data for a 577/500. Lots of other cartridges, but not that one, unless I'm looking at the wrong pages?

If you wanted to go duplex, I'd try 15 grains of H4198, then about 90-95 grains of FFg. Don't put much compression on a duplex load, just enough to stop the powders mixing about. You might need to increase the BP a little more to take up the space.

DC