Lamb and beef are sometimes combined together in shepherd's pie (particularly, it seems from several web references I've found, in 'British' pubs in the USA!). I can't see why they shouldn't be put together in a casserole.
Just because a combination sounds unusual doesn't mean that it won't work. Beef and Oyster Pie might sound odd to people who've never tried it but it tastes great. Similarly I suspect that prior to the 1950s hardly anyone in this country would consider putting pork, chicken and prawns in the same dish but nowadays millions of people order them together (in 'Special Fried Rice) from Chinese takeaways.
Be brave; give it a try!