Eric watched Lestat carefully. He could see the eagerness, but couldn't fault the other vampire for it. He was excited for this, himself. To see what would happen to them, if anything.
Lestat had mentioned penalties for sharing blood. Eric had never been put under such constraints. It was common for vampires to drink from one another if they were close, and during sex it was bound to happen even if it hadn't ever before.
Of course, if power transferred during this exchange, like Lestat had been speaking of, it would be easier to see why restrictions would be placed. He and Lestat were already powerful. If anything came of this, it would make them so much more dangerous.
He let Lestat drink for much longer than he thought was necessary, just in case.