>Oh yes, I forgot to mention that the paintings
>are 15"x22" and 18"x28" so are pretty good
>sized. They appear to be about 1/2 inch thick.
>
>The 18"x28" one has old cracks and was affixed
>somehow to a board and then patched. The colors
>do not match well and I was planning on re-doing
>that one when I bought the pair.

The idea is not so much to match the color as much as to "get rid" of white color of the new plaster.
Check the Sistine Chapel Restoraton book for pictures of how the restored portions should look.

> Seeing that the
>cracked one was affixed to a board makes me
>wonder if my first step with the newly cracked
>painting might be to affix it to a board to
>stablize it. But how? Epoxy?

Yes this would be the right thing to do. About using the epoxy - white portland cement slurry-like mixture will be a safer bet or check "Strappo Technique" for glue used traditionally for rmoving/relocating frescoes, however first you should determine if you are dealing with real fresco or not. You need to know what paster is used first.