I don't think Mercury Vapor bulbs put out enough heat to be the UVB and heat source because of the distance they need to be away to be safe.

I also think a 55 gal. is a mistake for a Beardie. They are too narrow (12" and are only half of the recomended floor space. A 75 gal. is just as long but 18" deep which is better and 50% more floor space. Or a 40 breeder is only 3 feet long but still has more floor space than a 55.

With a 75 or 40B you could use a linear UVB tube and a basking bulb. A 55 is too narrow for this.

The accepted minimum for a Beardie is 4' x 2' or 8 sq. ft.