I need to combine two wires with very light room lighting loads into one breaker in the panel to free up one breaker for another use. How best to combine the two wires and insert only one into the breaker (since the two can't fit)?
You can simply pigtail them in the panel - splice those two wires to a short third wire (pigtail) of the same gauge and type, and land the other end of the short wire on the breaker.
Wire nuts are most commonly used for splices, but they do take some skill to use successfully. Push in wire connectors are also commonly available, and are much easier to get right on your first attempt. In either case, you must be very careful to select connectors compatible with your wiring. In particular, be very careful if there's any aluminum wiring involved.
There is a persistent myth that the NEC prohibits splices in panels, but there's really no such rule.
You can also check the manufacturer of the circuit breaker to see if it is listed to have two wires per lug. Not all of them are, but there are a few very common brands that allow it. Square D QO series breakers for example are UL listed for two wires per terminal.
(I am assuming from the wording of your other posts that you are in North America somewhere so that the Square D brand name is relevant).
Also can you wire nuts three awg 10 together? Wont they get so thick? – Jtl
Are you unaware that wire nuts come in different sizes? There is a size for three #10 copper wires, e.g., Scotchlok (tm) G (the grey ones).
EDIT The listed sizes and numbers of wires to fit in a given wire nut does not take into account the thickening from "pre-twisting", i.e., the listed sizes and number of wires for a given wire nut presumes that the wire ends are just held parallel and the wire nut is twisted on until it is on hard tight.