I've already answered you on this matter. There is no need to understand a finite verb here. The utterance can stand on its own and be understood as a direct address/appellative or an exclamation. The entire utterance is compound grammatical subject (κύριός and θεός with modifiers), and the logical subject is Jesus.
Someone being God and not-God (i.e. man) simultaneously would hardly make for a "logical" subject. The "logical" identity of ὁ Θεός in John 20:28 is
the Father living in Jesus. Recall the last conversation Jesus had with apostle Thomas and another disciple in John 14 --
λέγει αὐτῷ ὁ Ἰησοῦς Τοσοῦτον χρόνον μεθ’ ὑμῶν εἰμι καὶ οὐκ ἔγνωκάς με, Φίλιππε; ὁ ἑωρακὼς ἐμὲ ἑώρακεν τὸν Πατέρα· πῶς σὺ λέγεις Δεῖξον ἡμῖν τὸν Πατέρα;οὐ πιστεύεις ὅτι ἐγὼ ἐν τῷ Πατρὶ καὶ ὁ Πατὴρ ἐν ἐμοί ἐστιν; τὰ ῥήματα ἃ ἐγὼ λέγω ὑμῖν ἀπ’ ἐμαυτοῦ οὐ λαλῶ· ὁ δὲ Πατὴρ ἐν ἐμοὶ μένων ποιεῖ τὰ ἔργα αὐτοῦ.
Here is Wallace NET Bible:
52sn Should Thomas’ exclamation be understood as two subjects with the rest of the sentence omitted (“My Lord and my God has truly risen from the dead”), as predicate nominatives (“You are my Lord and my God”), or as vocatives (“My Lord and my God!”)? Probably the most likely is something between the second and third alternatives.
He is on the right track with bold above, but the omitted words are not "...has truly risen from the dead," rather they are "...in him." In other words apostle Thomas said the following in John 20:28 -- "My lord and my God
in him !" .. [ὁ κύριός μου καὶ ὁ Θεός μου
ἐν αὐτῷ]. In John 20:28 Thomas finally believes what Jesus said to him and to Phillip in John 14:9-10 (
πιστεύετέ μοι ὅτι
ἐγὼ ἐν τῷ Πατρὶ καὶ ὁ Πατὴρ ἐν ἐμοί· εἰ δὲ μή, διὰ τὰ ἔργα αὐτὰ πιστεύετε.)
Anyone who stubbornly refuses to acknowledge that
ὁ Θεός in John 20:28
= ὁ Πατὴρ , will never get the true meaning of John 20:28.