I'm spending this summer trying to teach myself introductory Biblical Hebrew while reinvigorating my Attic Greek. I've been studying Greek on my own for a while now, so I'll read and transcribe several pages of Greek for practice and then turn to my Introduction to Biblical Hebrew. I can practically feel my brain switching gears; in comparison, it's very relaxing to read Greek for a while (despite its troubles), because I've already internalized the basic structure of the language.
As Chris says, Hebrew is a very different beast than Greek or most European languages, and I'm appreciating this more every day. My knowledge of Latin, Greek, and Romance languages is only useful to the extent that I'm used to memorizing and internalizing rules of grammar.
To second the others, I would choose one language and hammer it hard for a while until you've got the basics down. Then you can integrate the second language without being overwhelmed. I can tell you that you'll be happy not to have to memorize several disparate verb paradigms at the same time. Learning a new language is frustrating, and nobody likes to simultaneously deal with many frustrations.
Horae quidem cedunt et dies et menses et anni, nec praeteritum tempus umquam revertitur nec quid sequatur sciri potest. Quod cuique temporis ad vivendum datur, eo debet esse contentus. --Cicero, De Senectute