I would say no. Both languages will confuse in your mind (phonetically at least) and it will make you make mistakes that you would not make if you were studying only one of them. I have not studied Polish, but both languages come from the Old Slavonic branch of Indo European family, and I would say that it has one of the most complicated grammars that I have seen (and I have seen many). If you want to study more than one language at a time, you could learn without much problem two or more languages but from different families. At least far enough from each other so you do not mix them in your mind.

Better is to pick one of them, start to learn for a year - 18 months, build the language core, then pick the second. It would be less confusing and more effective. Russian and Polish are very similar, so you can confuse them. I know 2 person who are learning Polish and Russian at the same time. They confuse lots of words. Even if alphabet is different, sounds and some words are similar. I'm learning now Dutch. It's from the same family as English. My English is not that good, but when I have started with Dutch, English become harder for me. For example, during the lesson I confuse Dutch with English sometimes and it's hard to swich between those two languages. I can not imagine to learn German and Dutch at the same time, for me the similarity here is the same as between Russian and Polish.

I'd say yes, but you must concentrate! Grab a pen and write as you speak, and speak as you write. The different alphabets will be your best ally here, even if the words are phonetically similar. I agree with Mario in that similar languages are easy to confuse, but if you are disciplined enough you shouldn't have many problems.

Hi there, I have a similar situation - I've already learned Spanish for 3 years and some time ago I started with Portuguese. I think Polish and Russian have the same, let's say, level of similarity (they are REALLY similar to each other). If you start them both at the same time, I'd say there's little chance you won't get confused but if you try to learn one of them well and then, when you are fluent at it add another one, I think this would help you a lot. I can say so from my Portuguese - I needn't learn new words now, I usually only need to know how to modify them.

I would say no. Just learn one language at a time. I learned Polish a while back at http://preply.com/en/polish-by-skype so now I'm moving on to another language.