No, you can't. This card's 1st effect activates when your opponent has 10 or more cards in their hand or 10 or more cards in their trash, then the play cost will be reduced by 5. It doesn't activate when the total number of cards in their hand and trash is 10.

The play cost is reduced by 5. Even if they have 10 cards in their hand and 10 cards in their trash, the reduction to the play cost will be 5.