"You sit in the chair."
Translation:In sella sedes.
So, to be in the process of sitting, would rather be "in sellam".
But is there a plural if it's a "we" or a plural "you", and there's several chairs? Could in sellis and In sellas be possible?
Or is it a kind of fixed expression? (I think it is, and they consider that each person has only a chair)
But I've found "Ta semper illorum in sellis sedes", "ibi hospites dominus publius accumbunt mulieres in sellis considunt ", "Hebrais cum coenabaiit non in sellis sedere mos eral", etc...
So, I'm not sure now.