You need to get used to the FACT that Paul distinguishes God, who is the Father, from Christ who is the "Lord."
The Father is both God and Lord.
The Christ is both God and Lord.
And in fact, the term "LORD" is what the Jews used to call God instead of using the name, "YHWH", in order to avoid using God's name in vain.
But out of convention, Paul distinguished between the two by referring to God as "Father" (without denying His Lordship), and referring to the Son as "Lord" (without denying His deity).
And in fact, 1 Cor. 8:6 is an expansion of the Sh'ma, Deut. 6:4-5, which includes both the Father and the Son as the one true God.