I agree with Cloudchaser's answer. Because one book has the potential to increase the sales of another, you might want to let the first publisher know that you have any title coming out under the same pseudonym.
If you're having a publisher publish pseudonymous works, your agreement should specify under what conditions the publisher can reveal your identity. Sometimes, sales of books under a pseudonym can help sales under the author's real name and vice versa. Usually the author and publisher are on the same page about sales and promotions, but in some cases, there may be commercial reasons why a publisher might want to reveal the identity behind the pseudonym.
I'm assuming that the works are not part of a series, or if they are, that the publisher does not retain the option to publish additional volumes in the series. The initial contract should cover that stuff.