I have one of these thin dual circuit breakers Can I only

two breakers in one slot   Dan two slot shelving

Take a look at the photo below You can see that the back of these two breakers has two completely different slots These slots go into a bus bar exclusively

Some panels only allow one breaker per slot, while others allow slots to accept a tandem breaker, which fits two breakers in one slot  There are code limits on the number of circuits you can have in one residential panel And remember that both circuits are on the same phase

magic gate slot Remove one of the small 2 pole circuit breakers and put in a 30a or 40a 2 pole breaker and feed a sub-panel next to the current panel  One common version of a 42 slot panel in a smaller ETA the buss bar slot of the breaker itself was different between two breakers