So, a press is already considering your manuscript when another publisher approaches with interest in the same manuscript. Different editors may offer different advice on this question but, in general, honesty and openness are the best options. Publishers appreciate being told if there is a competitive situation as they may be able to address any questions you have about their potential contract offer in advance of the peer review process. Many presses do not allow simultaneous review of a manuscript, so it is best to ask your editor about their expectations as it can come to light when reviewers are approached. It can also cause issues when reviews and editors offer different advice and the author has to decide how to amend their proposal accordingly. Generally preliminary conversations with multiple publishers are fine but then you will need to select one to move forwards with peer review. If you don’t hear back from a publisher you could let them know that, if they are unable to confirm that they are happy to proceed with peer review, you will go ahead to that stage with another press at this point.
—Bristol University Press, February 2022
Of related interest: Can I submit my manuscript to multiple presses?