No more room in electrical panel, now what?

Please see comment below for information on how to combine 2 Double Pull Breakers to free

A tandem CB only requires one slot but would still count as two poles rmeier2: Maximum Number of Overcurrent Devices A panelboard

two breakers in one slot one energized wire and supplies 120V to a circuit They're generally rated 15 or 20 amps, occupy one slot on a breaker panel, and are the most common breakers

two slot toaster The duplex breakers provide two single pole breakers in the space typically required for only one standard breaker These duplex, 12-Inch frame breakers