my swag, the 2nd stage.
thoughts . . .
1) may have rotor imbalance/stability without 2nd stage impeller . . .
2) 1st stage may be design for low npsh - insufficient details provided.
3) you will likely be better off with pump suited for process conditions; than bastardizing existing pump.
frankly, i'd consult with OEM and obtain there opinion or expertise on the matter.
regardless, good luck!
-pmover