I would almost swear that the first time I ever played Oblivion, I could conjur more than one creature. Was that just a dream I had or is that something that comes with more skill than I have now?

It is not possible to summon more than one creature, although if you play on the PC there mods that allow you to.

Yeah if your on PC there are Mods and i believe a certain console command that you can put in, but it allows both you and the computer to summon more than one creature. So if you summon 2 monsters your opponent can summon 2 monsters as well.

If you have the Wizard's Tower DLC, you can use the Atronach Altar to create an Atronach Familiar and then summon something else via spell.

Maybe you had summoned a creature and one of the other contenders summoned the same type of creature, making you think that both creatures where summoned by you.

that's right! and if u are on the xbox360 or ps3 version, you maybe had a dream about it :/

Two things you can do who are not exactly summoning but pretty close. Closest is to get the raise dead spell from the ghosts at the hill of suicide in IS. It enables you to raise dead to fight for you. No limit on how many you can have however if your dead kills other npc and you raise them your undeads start fight each other. Not sure if the attacker continue to attack the target or the target want revenge. They also trails behind you making them of limited combat value.
High level command creature and humanoid is extremely powerful, it makes the target fight on your side for the duration on the spell. You face a pack of goblins; cast it on the warlord and he fights the others. More fun, unlike the frenzy spell if they hit each other multiple times they will become permanent enemies and fight each other after the spell is timed out.