lot of lizards species will get on fine with each other in the same enclosure for days , weeks or months. it only takes 1 litlle thing to set them off tho, and they will become very aggressive very quickly. i dont think it's really worth the risk to introduce another long term occupant to a beardie thats been alone for 2 years.
i have a pair that been kept together for years, since hatchlings, i put another of the same age /size in once, and the 3 lived together for about 2.5months. then one day i found the new one bloodied and bruised, with my female standing over him. it's not something i would do again