What size are the original two eproms? And what size is the new eprom.
I don't have the Simpsons board in front of me, but a Monday night football. In B there is a 27128 and in C there is a 27256. I am guessing the Simpsons is probably the same, assumption on my part as they are similar games built about the same time.

If they did not merge the program into a single eprom, then the size will be the same on each individual original dual eprom. If they merged them into a singl eprom it will probably be something like a 27512. It has to be a larger size if merged, if you follow. The person who sold you the eprom should be a good person to ask. If they sold you one eprom of a set of two, well, shame on them, IMO. These should be burned/replaced as a set for good measure.

Note, a lot or people re-flash crappy used eproms and re-sell them. These are usually the ones that go bad at a much higher rate, at least that is what I have found. It is extremely rare for an original eprom to go bad unless someone did something stupid to it.