hi, that form is clearly constructed, as the alpha-decl dual in the gen/dat isn't found in homer: see my notes linked above, s4.2.
pharr admits that this very form is uncertain: see s654.
other homeric grammars, e.g. chantraine 1958 chapter 17, thompson 1890 s17 &c., leave this part of the table blank.
the construction is based on the associated form of the other declensions, where the double iota is firmly attested in homer: see chantraine 1958 s81.
if you want to know what the physical evidence for the alpha-decl dual is (outside of homer), 1st place to look is inscriptions... see Threatte 1996 "The Grammar of Attic Inscriptions, Vol 2: Morphology", worth having on your shelf. it shows the single-iota form of the gen/dat dual was always used.
I've scanned the section for you:
http://www.freewebs.com/mhninaeide/Thre ... emdual.pdf
don't have a reference for papyrological morphology here at work, but that'd be the other type of physical evidence to follow up, cheers, chad.