Great question! It does happen sometimes that an author will let me know that their material is currently under review at a publishing house (usually because they had an open invitation from attending a conference), and that’s fine as long as they let us know. However, I wouldn’t suggest planning on submitting to agencies and publishing houses simultaneously while you’re trying to find an agent. Here’re a few reasons why: (1) It could be a deterrent to an interested agent if we learn it’s already been submitted to multiple houses (that’s fewer chances we have to get it sold). (2) Most agents are fairly hands-on editorially (I know I am!) and will work with you to make your project stronger before ever submitting to publishers. You only have one shot with a particular publishing house/imprint so you want your project in its best possible shape before submitting. Having an agent on your side BEFORE you submit will help you make it all the more strong and appealing to publishers. (3) When an author/illustrator (which I assume you are if you’re submitting a dummy rather than just a manuscript) meets an editor at a conference and submits a dummy, that author is typically only sending one project to one publishing house. An agent is able to send the project to several editors at one time in order to find the best match for your project. There you have it—three reasons why I suggest securing an agent before submitting your work to publishers.