They are both on Ancestry, I found them by trawling 
Reuben is transcribed as Reuben Dontfier, Elizabeth as Doutfire.
I doubt that there is another Richard / Sarah Doubtfire. Have you seen them on census anywhere.
Maybe Mary Ann's certificate will show a different maiden name for Sarah. If not I would be considering buying one of the certificates for a child born after the 1866 marriage.
I've been having such trouble with incorrectly transcribed names, it's made searching a bit of a challenge. I've found them both now and see what you mean.
This is very confusing. Having a marriage record for a Richard Doubtfire & Sarah Tyler, but a birth record where the mothers name reads as Sarah Doubtfire formally Earl. One the Doubtfire/Tyler marriage record it says that they were bachelor and spinster, neither widowed or divorced previously. So I'd discounted the fact that Sarah Tyler could be Sarah Earl.
I'm not sure what I do if Mary Doubtfire's birth record comes back with the mothers name as Sarah Tyler. I'm finding this rather difficult to make sense of.