Not only do I not see a problem with learning multiple languages at one time, I think it can directly benefit you in that something in one language may clear up a problem area you may have in another language. For example, my main language is C# and when dealing with LINQ I used the SQL format (which is fine, I’m not knocking it at all, but it just didn’t seem like the “cool way” to do it). I stayed away from Lambdas because, to be honest, I didn’t understand them and the C# documentation that I saw didn’t provide a clear definition (for me). Then I started to pick up some books on F# and started learning that, which gave me the “ah ha!” moment of understanding how Lambdas are.
I have found that as I have experienced more and more with other languages (and frameworks) I have become better at C# (and ASP.Net). That is why I believe that learning multiple languages at one time isn’t a bad thing at all!