The greek grammar for two persons. If you wanted to make it clear in Koine Greek that you are referring to two persons, and not one, ask yourself how would you say it?
Also consider the Greek at John 20:17, one definite article with four following nouns with possessives for the one and same person, God the Father. "The Father of me and Father of you and God of me and God of you." This is not the construction which Thomas used at John 20:28.
The Lord and God of me
The Lord of me and God of me
The Lord of me and the God of me
My Lord - He who has seen ME
My God - has seen THE FATHER
He who has seen ME - My Lord
has seen THE FATHER - My God