I got a shop-light fixture, leftover from a construction site. The metal boxes with 4 foot tubes. It has wiring for two lights in it - two of the small rectangular connectors on each end that you twist the tube into. This light is big enough for 4 tubes - it's 4' by 2', and I've seen them that size with four bulbs in the store. It has a black ballast box in the middle that shows how to wire it for two lamps.
So I also acquired another ballast wiring harness that shows wiring for three bulbs. Couldn't I install the three-lamp ballast box next to the two lamp one, put in the connectors and only use two of the three extra spaces? Basically turn the 2 bulb fixture into a 4 bulb fixture? The wiring shows that the three-lamp ballast will provide power "up to" three bulbs, but there's nothing saying I can't install just two of the bulbs and leave the third empty.
Would there be excessive heat build up from two ballast boxes being in there?