Except, it seems, the President can tell them to go home until needed.
...he may, on extraordinary Occasions, convene both Houses, or either of them, and in Case of Disagreement between them, with Respect to the Time of Adjournment, he may adjourn them to such Time as he shall think proper...
He can't prorogue the House or Senate unless there is a disagreement on adjournment between the two bodies.