Of course it is. Unless someone has posted that they require exclusive submissions they expect it. That is yet another reason why no one should send any submission without first checking the submission guidelines posted for the person they are submitting to.
Depending on the place we are submitting to it can take from a few weeks up to a year or more to get a response on a submission. The most common response time is around 90 days. I don’t even follow up on anything until it hits the 90 day mark.
If we had to wait on each submission until we heard back on another one and had a minimum of 90 days between them it would take forever to find a place for our work. Editors and agents know that and know multiple submissions have to be made. On those occasions that I am dealing with someone who requires an exclusive read I will only do so for a defined period of time, not to exceed six weeks.
There are some courtesy rules involved in multiple submissions, however. First, submissions should never be made to more than one person in the same agency or publishing house at a time. That can cause problems.
Submissions should be personally addressed even though we are sending to multiple people. Many agents and editors feel if someone is not paying them the courtesy of personally addressing it, they don’t owe the courtesy of a personal response.
“Dear Sir or Madame” letters are generally regarded by editors and agents as “Dear Occupant.” What do you do with your occupant mail? We do too.
Submissions with dozens of email addresses showing in the TO box tend to be regarded as spam. Everybody can wind up looking at the list and thinking “Well, somebody will answer them.” And nobody ends up doing so. Dozens of rejections by omission occurring just because someone was too lazy to individually address them.
Back on the submission guidelines. I did a survey of agents and editors a while back for an “editor and agent pet peeves” course that Linda Glaz and I are both offering at conferences. The number one pet peeve, the thing mentioned the most was not following the submission guidelines. There are a number of things in submissions that can be a problem, but most of them can be avoided by looking up and following those guidelines when making a submission.
Can more than one agent or editor be contacted at a time? Absolutely, as long as we are smart and courteous in doing so. And in looking at those guidelines please not that some want to be informed if it is a multiple submission. Most of us just assume that it is.