Gabe,
Do we not follow the same precept; we place the sign as they did; we place the sign on new converts, as they did. As far as the supper goes, it is a refreshed antitype of the passover-no?
The question to ask would be is the trinity a requirement that is foundational in the gospel message? Jesus is God right; one needs to have this onboard before they can be converted. That would account for the two members.
Gen 1:2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
Exo 31:3 And I have filled him with the spirit of God, in wisdom, and in understanding, and in knowledge, and in all manner of workmanship,
Num 24:2 And Balaam lifted up his eyes, and he saw Israel abiding in his tents according to their tribes; and the spirit of God came upon him.
Jdg 3:10 And the Spirit of the LORD came upon him, and he judged Israel, and went out to war:
Jdg 6:34 But the Spirit of the LORD came upon Gideon, and he blew a trumpet; and Abiezer was gathered after him.
Mat 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
I believe that the OT saint was fully aware of the trinity and it's position in the doctrine of God.