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As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

and pyrophosphite:

$$\ce{^{-}O-PH(=O)-O-PH(=O)-O-}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:

$$\ce{HO-PH2=O}$$

Pyrophosphite is not analogous to pyrosulphite, nor phosphite to sulfite. There is no $\ce{PO3^3-}$ nor $\ce{P2O5^4-}$.

Pyrophosphorous acid is in this context similar to acetic acid. Not all their hydrogen atoms are acidic, so even if fully neutralized, some remain. Sodium acetate is $\ce{CH3COONa}$, not $\ce{CNa3COONa}$.

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

and pyrophosphite is:

$$\ce{^{-}O-PH(=O)-O-PH(=O)-O-}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:

$$\ce{HO-PH2=O}$$

Pyrophosphite is not analogous to pyrosulphite, nor phosphite to sulfite. There is no $\ce{PO3^3-}$ nor $\ce{P2O5^4-}$.

Pyrophosphorous acid is in this context similar to acetic acid. Not all their hydrogen atoms are acidic, so even if fully neutralized, some remain. Sodium acetate is $\ce{CH3COONa}$, not $\ce{CNa3COONa}$.

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:

$$\ce{HO-PH2=O}$$

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

and pyrophosphite:

$$\ce{^{-}O-PH(=O)-O-PH(=O)-O-}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:

$$\ce{HO-PH2=O}$$

Pyrophosphite is not analogous to pyrosulphite, nor phosphite to sulfite. There is no $\ce{PO3^3-}$ nor $\ce{P2O5^4-}$.

Pyrophosphorous acid is in this context similar to acetic acid. Not all their hydrogen atoms are acidic, so even if fully neutralized, some remain. Sodium acetate is $\ce{CH3COONa}$, not $\ce{CNa3COONa}$.

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

As phosphorous acid is biprotic:

$$\ce{HO-PH(=O)-OH},$$

the respective pyrophosphorous acid is biprotic too:

$$\ce{HO-PH(=O)-O-PH(=O)-OH}$$

Therefore, potassium hydrogen pyrophosphite would be:

$$\ce{KHP2H2O5}$$

respectively

$$\ce{KH3P2O5}$$

By other words, from 4 hydrogens of pyrophosphorous acid, only 2, bound to O, are acidic. The other 2, bound directly to P, are not.

Similarly, hypophosphorous acid is monoprotic, as only 1 of 3 hydrogen atoms is acidic:

$$\ce{HO-PH2=O}$$