I read a lot about Knife Fish (Badest Fish IMO) and everywhere I read they say that they are agressive and territorial and should be kept alone, but I have yet to meet an aggressive Knife fish. Go for it! They're cool!
I've seen a lot of pairs of Clown Knives and pairs of Royal Knives, but never a mix. I actually tried to have a pair of Clown Knives, but I introduced a new one to a tank that already had a CK living in it for about a month. The poor little thing never ate and didn't survive. I'm not sure if the other Ck had anything to do with it or if it was just a weakling. So you can try it out, but keep an eye on them. Good Luck