User blog comment:Omega natsu2/Episode 179: Gray vs. Rufus, Review./@comment-288216-20140426233755/@comment-7418318-20140427082009

His ability to combine any spell in his arsenal with any other in the double combination already completely fulfills his statement.

He never said he can combine as many spells as he likes into a single one. Considering how the only cases of spells from multiple kinds of magic we've ever seen were the one case of Abyss Break and the cheat-mode Dual-Mode Dragon Slayers, I think casting two kinds of magic takes more than just the necessary imagination and that it becomes harder with each new kind of magic you add into the mix.

Disregarding the DS for the moment, since their magic is different from regular magic, Rufus is one of ONLY TWO mages in the entire series who was shown to use two different elements simultaneously. Even people like Erza, Mirajane, Laxus, Jura, Bluenote, Gildarts, Hades and Makarov havn't done that on screen yet, so I think just combining two elements is already an amazing feat. Especially for someone who is neither a guild-master, nor a wizard saint nor even considered his guild's ace.