Still scratching my head a bit. Seems to me that if your book was published and successful, your publisher would be eager to consider a sequel. If your book was published and did not meet whatever the publisher's expectations for it were, it's likely the publisher would not want to publish a sequel. But I'm not seeing what the publisher's interest would be in stopping you from offering the book elsewhere, once they turned it down.
frimble3 points out correctly that the more likely problem is that it would be be hard to find someone else willing to publish a sequel, especially if the first book published by the original publisher didn't succeed.
And lizmonster is correct that you have control over whether or not you agree to the terms in the original contract.
The best thing you can do, once you are offered a contract, is to have a lawyer experienced in publishing go over the contract with you, so you know exactly what you are agreeing to if you sign it.