I've used splitters many times before and never had any ill effects as a result of doing so. Sadly, that was the only way to do things back in the good old days when most radios came with ONE set of RCA outputs.
In fact, I am two Monster Cable splitters in my Mustang right now because I had to break the signal between front/rear fill, front midbass, and subwoofer with my deck only having 3 sets of preamp outs. I didn't mind splitting the front output between the front speakers and the rear fill because my rear fill has been gained down to the point where I can barely hear them. The added plus was the front to rear fade gave me control over the level of the 8" midbass drivers in the doors since I stuck them on the rear outputs. The subwoofer, well that bad boy is on its own!