Short answer: Yes. Both can withdraw. Both must qualify individually. Both must intend to live in the home.
Yes. Both partners can withdraw their KiwiSaver balances toward the same house. Both must qualify individually. Both must have been in KiwiSaver for 3 years. Both must intend to live in the home. The money goes to the same lawyer for the same settlement.
Partner A Balance: $25,000
Partner B Balance: $18,000
Combined: $43,000
Toward House: $600,000 purchase