In post #1 the OP is wanting to install one T-8 ballast for two new T-8 lamps and install T-8 lamps on the other good existing T-12 ballast not putting T-12 lamps on a T-8 ballast.
The problem I have seen is even if the T-12 ballast is rated for the two F32 T-8 lamps they will not burn at the same brightness or color can be off, I have also seen that the T-8 lamps do not last as long because they are being hit with a higher voltage that it takes to ignite a T-12.
Any time I get a ballast failure today since T-12s are not manufactured anymore (only existing stocks are being used up) that it very worth while to just upgrade the fixture to all T-8 lamps, F-32 watt T-8 lamps will generally produce more light, the newer electronic ballast do not use high voltage to ignite the lamps, they use high frequency (25kz) to do the same job with less energy, another nice feature is they are mostly a parallel ballast which means if one lamps fails the rest of the lamps stay lit, the wiring of the newer electronic 4-lamp ballast are much simpler as they only have five or six wires to the lamps depending upon the manufacture, most Sylvania/Motorola electronics have six wires on their electronics others have five, two yellows, two reds, two blues as seen in this diagram:
The 5-wire 4-lamp ballast just has one yellow for all the yellows at the one end.
converting it only takes connecting both wires at each lamp ear together then connecting it as shown in the diagram on the ballast or above, just make sure you match the blues to the same two lamps you connect to the one yellow and the same for the red.
A two lamp electronic T-8 ballast will only have 3 wires going to the lamps, either one red and two blues or two reds and one blue, the wire color that is only on one wire goes to the older yellows and the two wires of the same color goes one each to the other end of the lamps (old red and blue wires) as shown in the diagram below:
trust me 6 wires is allot easier to wire them the 10 that older 4-lamp ballast had, or three wire in the case of a two lamp.
Another nice feature is a 4-lamp electronic can also be used for a 3-lamp fixture by just not using one of the red or blue wires, and a 2-lamp ballast can replace a single lamp ballast and most of them now can be supplied from 120v to 277v will allow you to only stock two types on the truck and cover almost all 1-lamp to 4-lamp fixtures that are on any voltage from 120v to 277v which is what I had started doing on my van, I would keep 4 of each type on the van for service calls.
But as others have said, for the little extra cost of a 4-lamp ballast I would just install a 4-lamp electronic so at least you wont be called back because of the difference in light output that running two T-12's on one type of ballast and two lamps on the older T-12 ballast, just tell the customer that T-12 lamps and ballast will be disappearing from the shelves soon anyway so it would be better to just upgrade both now with a single 4-lamp ballast, trust me the extra cost of a 4-lamp over a two lamp is not that much more, at one time Menard's had them priced the same.
