You can’t choose to exile just one creature card.

The two creature cards may come from the same graveyard or different graveyards.

You choose which one The Mimeoplasm is copying and which one determines how many +1/+1 counters are placed on The Mimeoplasm as The Mimeoplasm enters the battlefield.

Treat The Mimeoplasm as though it were the creature card it’s copying entering the battlefield. Any “As [this card] enters the battlefield,” “[This card] enters the battlefield with,” and “When [this card] enters the battlefield” abilities of that creature card will work.

Specifically, if The Mimeoplasm enters the battlefield as a copy of a creature card that enters the battlefield with a number of +1/+1 counters, The Mimeoplasm will enter the battlefield with those +1/+1 counters and +1/+1 counters equal to the power of the other exiled card.