I laid out all the lighting and wiring around the house. Went out and bought the biggest transformer HD had - 600 watts. I get home and find out the 600 watts is actually two 300 watts transformers. The lights are very dim.

Can I wire the transformers in series to get more watts to brighten the lights?

My lights add up to almost 400 watts and I used the heavy wiring. 600 would do it.