On the Norris/Lee issue: Chuck Norris is one of the greatest fighters of all time, there is no doubt. He is the only person in the western hemisphere to hold an 8th degree black belt in tae kwon do, awarded in Korea to him. He was 6 time world middlewieght champion, undefeated. He never 'fought' bruce lee. He trained with him, and was in Return of the Dragon with him at Bruces request. When asked what bruce was like he said
'Very, very, VERY fast. He was also as strong as I was, even though he was 25 lbs lighter. He learned everything, used what he thought best, and made it his own. When I first met him Bruce never kicked above the waist, he didn't see the point in it. After working out with him and discussing it with him he began to include high kicks, and within a few months he was as proficient in high kicks as anybody in the world. He was really one of a kind.'
I think it's safe to say that if they ever had fought, it would have been epic. It definately would NOT have been a one sided ass-kicking though.
On the matter of training in multiple arts: Bruce Lee is probably the best study of this. His 'Jeet Kune Do' was an amalgomation of ALL fighting styles in the world. However he didn't just pick up a little here, and a little there. He studied an art in depth, intricately...coming to understand it fully, before moving on to the next art.
It also depends why you're taking them. Tournament fighting, self defense, fitness, spirituality? There are good reasons to take more than one, and good reasons to stay focused.
Jujitsu and Karate are definately different styles, so there's not likely to be 'conflict' as such...especially if we're talking about a small circle jujitsu or aikido derivitive. However, the arts have a different general 'feel' to them, so it might be difficult to switch modes, which can throw off your timing and execution.
My personal suggestion, having taken classes at 7 (I think) different schools in my life, is to decide what you want, and why. Then decide which art most closely fits that desire. Learn that for a minimum of 2-3 years, as long as the school is good. After that you should be comfortable enough to begin branching out and adapting other styles.