It's actually probably easier to learn two unrelated languages at the same time than two related ones. You're more likely to get two related ones confused, and I think you're more vulnerable to it when you don't know either one well. Though I remember one time I was quizzing myself on Japanese and I couldn't think of "credit card" -- How hard could it be? It's "kurejitto kaado"! -- for a few seconds because the phrase "tarjeta de crédito" was blocking it in my mind, even though I knew immediately that it couldn't possibly be Japanese. So far this sort of thing rarely happens, though, but I don't know all that much Japanese yet.
Oh yeah, that's another thing. Learning multiple languages sucks up an insane amount of time.
But at least you can always fall back on the other one for a while if you get bored with one. (I've stopped getting bored with Spanish, for the most part, so eventually that problem goes away by itself, if my experience is representative.)
I am learning Spanish and Italian at the same time, but it's not so crazy for me because I've got a few years of Spanish under my belt before starting Italian. (I'm also concentrating on Spanish much more at the moment; I'm still in the "testing the waters" stage with Italian.)
Just looked at my previous post in the thread. Interesting that I was still "just finishing Heisig" (I finished it a couple months ago) and flirting with Italian instead of studying it. I did end up starting Italian before really finishing any of my Spanish books, but that's largely because I realized my level of Spanish really is pretty good. Still so very far from native-like, and I wouldn't be a good candidate for an English->Spanish translation project, but good nonetheless. (A Spanish->English project, on the other hand, would be quite reasonable as long as it wasn't loaded with cultural references I wouldn't get.)