>Latin has a "double ablative", a dependent construction where a
>subject and a past pasticiple (*-tó-) of its verb are given ablative
>endings.
I presume you mean the ablative absolute. The origin of this is unclear,
but it is clearly a true ablative, as it is found in Greek as a genitive
absolute. It is also likely to have begun with noun + noun (e.g. "Caesare
duce"), The nominal origin is also seen in the -e ending of the present
participle (rather than the adjective -i ending). The subject and past
participle (*tó-) construction therefore may not be inherited.
Peter