I think my 9.3 loads barely make 2100 fps with a 286 grainer. But they still did the job.

I don't see any purpose at all for a .378 Weatherby except to make powder and barrel makers rich. If one wants increased big bore killing power I much prefer to go up a calibre or three.

Did Weatherby make one in a double?

No for increased range and a flatter trajectory in a double for plains game a .300 H&H or similar or maybe a 8x75R has their attraction. Even the set of .30-06 barrels I have waiting for me.