Dedicating time to one art will always impede your progress in another.
I would reccomend not doing multiple arts until you are atleast Shodan in your primary art, maybe even waiting until Nidan.
Then, find an art the complements your main style well...
Boy, that's pretty conservative advice. Well, then again, I
was the equivalent of "nidan" in Wing Tsun before I took up my second art, Escrima. But if you choose a second art that is compatible in
concept yet sufficiently different in
application (say standup, plus grappling, or weapons, etc.) you should be able to avoid too many conflicts. And of course you may learn each a bit more slowly, since you are dividing your time.
Still, it's simply innaccurate to say that studying one art
always impedes your progress in the other. As others have already noted, you may find that some cross training really opens your eyes. Sometimes I've found that something I learn in one art suddenly helps me understand a core concept or idea that I had been missing in the other. Then "snap" it all comes into focus. Kinda like the time I invested in learning Spanish. It actually gave me a better understanding of English, too. So with the martial arts, if I had time, I wish I could have studied a third system...say BJJ or similar grappling art. Of course, I'd also like to speak a half a dozen languages...but it ain't gonna happen. So I'll be content with WT and Escrima...at least for now.