There’s nothing incompatible between saying that there are two Covenants, and that the Christian Covenant is the only Covenant of Salvation. The Hebrew Covenant is upheld by God, as can be seen by the Jews’ continued existance. The Christian Covenant is a different kettle of fish, a Covenant of Salvation for all people, Jew and Gentile.
The Hebrew Covenant doesn’t really speak of Salvation in any detail, only that it is coming. There are vague hints of what the Kingdom to Come will be like, and various expectations of the Messiah, but they are pious speculation under the Hebrew Covenant. The only sure thing is that the Messiah will indeed come, and will usher in the Kingdom to Come for both Jew and Gentile. The Christian Covenant is nothing more nor less than the fullfilment of that aspect of the Hebrew Covenant, the bringing of Salvation to both Jew and Gentile. The existance of the Christian Covenant does not erase the Hebrew Covenant, but rather builds upon it, and the Hebrew Covenant is not predicated on the acceptance of the Christian Covenant, as it is built on the faithfulness of Abraham and the promise God made to him. The Christian Covenant is instead the ultimate capstone on the Hebrew Covenant, new in that it applies to all people rather than just the Jews, and old because it grows entirely out of the Hebrew Covenant.
We can see from history that God has upheld the Hebrew Covenant despite the failure of many Jews to accept the Christian Covenant (and even to uphold their own faith in many instances), and Jews have been protected from annihilation in spite of concentrated efforts to wipe them out. Clearly the Hebrew Covenant still stands, as it was made with Abraham and predicated on his faithful obedience to God. In fact, to say that the Hebrew Covenant is void is to call God a liar, because He promised Abraham that all his descendents would be looked after, and would become “as numerous as stars in the sky”. God did not deal in Salvation with the Hebrew Covenant, so neither is it incorrect to say that only Christianity is a Salvivic Covenant. The Hebrew Covenant and the Christian Covenant exist side by side, with the Christian Covenant “sprouting” from the Hebrew Covenant. We are always warranted in hoping and praying that Jews accept the Christian Covenant, and even in friendly invitations for them to come and claim the reward that God promised them first. Gentiles are essentially guests at the banquet promised by God, we are the wild branches grafted on to the Tree of Life, and it is only right and proper that we should hope for the guests of honor to come to their own feast that God has prepared and promised. Honestly, it seems kind of empty without them. As Paul said, how much greater will the Tree be when the native branches return?
The problem is that for many centuries it was popular to view the Hebrew Covenant as null and void, which is absolutely preposterous. Jesus spoke of the New Covenant, certainly, but did not say that the Old one was being wiped away. Paul elaborated and said that the non-Christian Jews would be preserved until the full measure of Gentiles came into the New Covenant, at which point they would realize God’s faithfulness to them in the Christian Covenant and come in themselves. The Scriptures clearly point to the survival of the Hebrew Covenant until the end of time, even as the Christian Covenant is the one that Saves.