Simultaneos submissions

As one who spent the last few months querying agents on a YA paranormal at the query stage no, but the ones who wanted o see more asked that I let them know if anyone else was looking at the book.

Barb
who only had to endure 5 "nos" and 4 non responses before getting the "Yes"

What Barbara said. No need at the query level, and you probably don't even need to if they only ask for a partial. But it's good manners to be up front about it if they ask for a full ms. If anyone requests that you give them an exclusive look, do so at your discretion, but give them a time limit (say, two-three months) so it's not out there hanging forever. Then, when the time period is almost up (a week or two before), send a friendly letter, reminding them that the exclusive look has nearly expired. If you still haven't heard back at the end of the time, don't hesitate to send it out to others who have asked.

Oh, and if you're asked for a full by another agent if/when the first one has it on an exclusive, you can simply tell the second agent that someone has it on an exclusive and that if you're not offered representation, you'll be pleased to send it on XXX date. That's usually fine with them. If you do send it to them later, make sure you send along a copy of the request letter/email in the package and mark "REQUESTED BY [INSERT NAME]" on the outside of the envelope in big letters. That way, they won't have forgotten about you in the intevening couple of months.

Good luck!