Sarah,

After researching it a little more, beyond their appearance in the 1910 US Census, I found Mr. & Mrs. Nathaniel E. Whittier multiple times in the Arlington MA Directories from the early 1900's up until 1918. After that they disappear and as you have mentioned I can't find either one in later censuses. It would seem unusual that both passed away between 1918-1920?

Rick