Thanks to Martha Kelly for the clues. She has a Montgomery cousin, Lillian Campbell Brewer, who told us that Allen and Martha Montgomery are in Ryders Hill Cemetery in Lebanon.
Dates: Allen b. 3-12-1818 and d. 01-15-1895
Martha b. 11-19-1829 and d. 6-18-1889
Lillian owns the old Montgomery family home in Lebanon called Myrtledene.
Allen and Martha's daughter Lily Montgomery (who married Tom Jackson) lived there, as did her daughter Lillian and HER daughter Lillian. Lillian Campbell Brewer still owns the house, though it's now a B&B.
If someone in the area would volunteer to take photos of the two grave stones for me, I would be eternally grateful.