This is in the singular, referring to one of the gardens, as already in the previous ayah: jannatahū.
مِنْهُمَا
minhumā
“something even better there”
This is in the dual, referring to the two gardens.
Explication
These readings amount to the same meaning. The latter accords with the codices of Makkah, Madinah and the Levant with an additional mim to form the dual [Makki].
He has revealed to you ˹O Prophet˺ the Book in truth, confirming what came before it, as He revealed the Torah and the Gospel
Explication
These readings amount to the same meaning. The latter accords with the codices of Makkah, Madinah and the Levant with an additional mim to form the dual [Makki].