Some publications, journals, publishers and agents are happy for you to send your work to multiple places at the same time, while others are not.
As a courtesy, always let the editor or publisher know in your pitch email or cover letter if you have submitted the manuscript elsewhere.
While literary agents are usually happy if you’re submitting work to other agents (as long as you tell them), they may be less likely to consider your proposal if you’re sending your work to publishers at the same time, as iit can be difficult for them to negotiate contract terms after the fact.