I'm in the process of learning Greek with an on-line tutor and let me tell you it's damn hard work. For a native English speaker learning Greek is like learning Klingon - almost impossible!
In English we have only one gender for everything but not so in Greek also Greek grammar is impossibly complicated and without a first class teacher you're almost certain to fail and fail spectacularly.
With that in mind and still having some spare time I thought why not learn two languages at the same time after all how hard can it be?
I thought that as Russian has similar language roots it would make an ideal second language to learn and the similarities would help me in my quest.
Fail!
Oh my God Russian is even more complicated than Greek and speaking it makes your brain hurt! Also I hadn't considered the extra cost involved in having two language tutors :(
To cut a long story short I've ditched the Russian and devote all my time to learning Greek I haven't given up the idea of learning two languages but I think one at a time is all I can manage at the moment.
Urban Legend has it that learning your second language is the hardest and once you become fluent in two languages the third is easier and the fourth even easier still - I'll let you know :)