It depends on many factors, like how much time you spend on each one, how related they are to each other and to your native language, how good you are at remembering new words/grammar patterns etc. In my own experience, I find it easier to learn languages that are closely related, because if you study one grammar pattern, it is quite likely that the other language(s) uses it in more or less the same way. Also the vocabularywill be quite similar: morte, muerte, mort, mors[mortis] are very similar in italian, spanish, french and latin. My native language is spanish, and I have been learning Italian, Latin and French with no problems (no more than the usual, that is) But as Jura said, you may mix up languages, and if you do not practise enough, you'll find it quite difficult... I know that from my own experience too :( But go ahead and try. If you are constant, you'll be speaking those in a year or two. ;)

I think it depends on you after all , Learning two languages together could be great , like if you learn a lesson you'll be able to find the differences between the 2 languages , which will make you memorise faster. But if you're a person who could be distracted very easily like i am , then i'd suggest you not to :D

it is probably better to concentrate on one language when starting then as you get the feel of the language you can apportion some of your time learning the other language that interest you.

No, I mix up my Italian and French words.