$HPO _{4}^{2-}+ H _{2} O \rightleftharpoons H _{2} PO _{4}^{-}+ OH ^{-}$
$HPO _{4}^{2-}+ H _{2} O \rightleftharpoons PO _{4}^{3-}+ H _{3} O ^{+}$
$H _{2} PO _{2}^{-}$ is a conjugate base of $H _{3} PO _{2}$ (a monobasic acid) and does not give $H ^{+}, HPO _{3}^{2-}$ is a conjugate base of $H _{2} PO _{3}^{-}$ and does not ionise further. Since, $H _{3} PO _{3}$ is a dibasic acid.