There is an old word, _whether_, used formerly to mean which of two,
but now obsolete. Examples from the Bible:--

_Whether_ of them twain did the will of his father?

_Whether_ is greater, the gold, or the temple?

From Steele (eighteenth century):--

It may be a question _whether_ of these unfortunate persons had
the greater soul.


[Sidenote: _Use of_ who _and its forms._]

98. The use of _who_, with its possessive and objective, is seen in
these sentences:--

_Who_ is she in bloody coronation robes from Rheims?--DE QUINCEY.

_Whose_ was that gentle voice, that, whispering sweet,
Promised, methought, long days of bliss sincere?--BOWLES.

What doth she look on? _Whom_ doth she behold?--WORDSWORTH.

From these sentences it will be seen that interrogative _who_ refers
to _persons only_; that it is not inflected for gender or number, but
for case alone, having three forms; it is always third person, as it
always asks _about_ somebody.