[Correlli]

Just add the same coordinates and same interiorid for both houses. And virtual worlds are useful in house-system.
If two players are in the same interior/place and if player1 has virtual world different than player2 then they can't see each other.

https://sampwiki.blast.hk/wiki/SetPlayerVirtualWorld
https://sampwiki.blast.hk/wiki/SetVehicleVirtualWorld