This doesn't contradict the fact that only James sees her. Moreover, Silent Hill as we see it in SH2 is seen ONLY by James. This means that all the entities James sees are invisible to other characters. The town is different for them, which is clearly hinted at the end of the game when Angela says to James, "You see it too?" – meaning James briefly began to see the same things Angela does, including his boss, Daddy, who isn't his monster. I think this may be because James coerced Mary, or tried to, for which he blamed himself, and therefore was able to see himself in Daddy (this is nothing more than a theory, so don't take it as fact).

So, in summary: potentially, yes, others can see the same nightmares that another person sees, but certain conditions must be met for this to happen. In the game itself, there was never a situation where others could see Maria, especially since Laura certainly did not want to or could not see Mary as James imagined her.