When the client generates the zaps
, it would just lock half to one npub, and the other half to another (assuming 50/50 split)

So the client wallet would take X amount of ecash and request the mint to create new ecash tokens locked to the proper npubs, and then you just sent those along with your zap note. 
 

 The receiver could just look for ecash locked to its npub, and then claim it immediately and add it to a wallet. 
A couple of web based cashu wallets do this currently using nostr DMs