You pretty much hit the nail in the head EF. Sometimes you'll find 2 suitable races that suits the horse and is elligible for so you nominate them for both. Nominations for weekend races usually have to be in by about noon on the Monday before and final acceptors for the trainers who decide to run their horse in any particular race have to be announced by 9am on the Thursday morning.
it costs nothing to nominate your horse and you only pay if you accept for the race so that's why you might see races with 50 or more nominations, trainers will put their horses in case because you can always pull them out by acceptance time and cost you nothing. The reason why trainers will accept for 2 races is because they don't/can't find out who they're racing up against until the Thursday morning so they'll wait and see which race would be more suitable in regards to the strength of the field generally.
As you mentioned we dual accepted the other Friday night and the next day. We wanted to see which horses accepted for both races and which one we thought we had more of a winning chance in. If both races were on the same card we'd still have done exactly the same thing, we still had to pay both acceptance fees but that was better than to guess which one will be the more suitable only to choose the wrong one to run in.
Anyway i hope that helps a bit.
