If I understood correctly one of the comments by @AlisonBabeu on #2740 , the aim was to combine supplementary parts that do not constitute a separate edition to the same files:
"There are a few places where URNS have been used inconsistently and 1st1K-grc2 or 1st1K-lat1 have been used to represent supplementary parts of a printed edition (say a preface, intro, index, etc.) but I've been changing those as I find them so we are intellectually consistent across the whole collection. In general, if there is a 1st1K-grc1 and 1st1K-grc it means there are two editions of a work."
In tlg0643.tlg001.1st1K-grc2.xml there are only the sigla from the same edition as tlg0643.tlg001.1st1K-grc1.xml. It's short, not very informative without the actual siglas and does not constitute a separate edition, so I see no reason for it to exist separately like the other edition texts.