Simultaneous submissions is a very common approach to publishing. Instead of sending out a project to one location, wait for a response, and then move on to the next, we often send a project out to several people at one time. While agents and editors have no problem with this, it is important that there are protocols authors need to follow. I am bringing this up because I have been hit with multiple authors in the last month who "must have missed the memo" about these protocols.
First of all, authors should not be using the multiple submissions as a lever to pressure editors and agents into signing them, or getting them to move faster. Remember, we know what we are looking for and your story will have it or it won't. I would also remind you that your project is not the only thing we are dealing with. We have other clients, other projects and a method for how we deal with submissions. Don't push it.
Secondly, if you are sending out your manuscript to, let's say 5 agents, you need to know that ALL 5 of them are on your wish list. Telling that author, after they have made an offer of representation that you want to see what the other people have to say first is simply telling that agent that you are NOT a top priority.
I would also recommend considering your simultaneous submissions in small blocks. Try three to five and see what happens. If those get rejections, review the comments and then send off the next round. I think a lot of authors just keep sending those projects out before they hear back from some people. Now they put pressure on the editor or agent of the latest submissions to move faster. Not going to happen.
Communication is the next piece of the puzzle. If someone does offer representation, let everyone else know. If I still have to read your project, you can save me the time and energy if you are already going somewhere else. I promise, it does not hurt our feelings.
Finally whether this is a simultaneous submission or a single submission, give the editor or agent time. I had one author several months ago submit a project to me, and then four days later, was already emailing me asking if I had gotten to the project yet. For the most part, three months for a response might be what you will be looking at. Read what the editor or agent says for a turn around and stick to that!