I don't see any issue with keeping two carpets of different color together given they're both healthy and that you're running carbon. The issue is when you introduce newly shipped anemone with possible internal infection/virus in the same system as your existing anemone(s). You have to be selective as to what to add. If the anemone you want to add is from an existing system and doing well you still have to be careful and run heavy on carbon and maybe a small water change the next day or two.
After I got my first gigantea to a stable healthy state, I added the red haddoni only because it has been in the lfs for over a year and I think I can provide a better home. There was no issue there. Then I added the other gigantea after I saw a member of my local reef club posted that he's breaking down his tank and the gigantea has been in his tank for about 8months. The pictures he posted looked good so I pick it up. No issue adding it either. Recently, the sunburst/flamethrower BTA craze went crazy and I saw some really nice BTAs at my lfs that has the potentially of being the prized sunburst under right lighting. They were all bubbly and has nice colors to them so I bought two.
Initially, my red haddoni was sensitive to the new addition. It started to shift slightly out of it location. I did a water change and change my carbon and it's staying put now.
So in summary, I don't think you will have a problem with mixing anemones as long as your system is ready for it and that you know what to add and when to add. Carbon, IMO is the key with early water change in the early stage of introducing the new nem.