Other rankins dragons... that's it! Anything else and they might fight like billy-o, resulting in two seriously stressed and possibly dead reps!! I it's mae get him some girly friends as he will fight with males; if female, you can get either. Also depends on your viv size.
beardies, rainkins or not, are not solitary animals, and get stressed out with 'friends' around. male/male will result in fighting, male/female will result in overbreeding and stress on the female, female/female is the only combination that may work, but its still risky.. its best to house them separatley...
i seen tortoises mixed with nile monitors and rhinosaraus iggys at blackpool zoo. dunno if they suitable for mixing with rankin dragons but u'd need a massive enclosure to do it any so both species have their own space.
I still dont get why people want to mix thier species. While Im quiite happy sharing my home with other people I dont think Id be very happy with a cow walking around in front of me while im watching tv :?