I've surrendered after a split in Spanish 21 (8's split to give one 17 vs A). I would assume that this would be against the rules since you are only suppose to surrender on your initial two cards. I haven't ever seen a rule in Spanish strictly prohibiting this, though.
Since the request to surrender after a split is pretty rare, act confident when you surrender and there's a good chance the dealer will do it without thinking about it. I've surrendered on a 3 card 17 using this tactic as well.