There is information about entering pet class here:
Preparing for the Pet Class
You don't need to separate them. If you wanted to enter just one, then you could ask the show sec to pop a note on the pen label you will be given on entering to identify which hamster you wish to be judged (e.g. black/normal), or if you enter both you will be given two pen labels.
You enter them on the day. Pet class is judged at two, so it's worth arriving at 1.30-1.45 to give you enough time. You speak to the show secretary who takes your name and the entry fee then gives you the pen label(s) and directs you where to put the hamsters.
A carrier is good. You should give them some bedding, some food and a moisture source (either a bottle or some vegetable). You can put toys in the carrier unlike in main class.
For Midland shows, you can only enter pet class if you are a junior member, a non-member or an adult with fewer than 10 hamsters