If your MeshInstances are facing the negative Z axis, then you can use the look_at function to rotate the mesh so it looks at a given point:
$Human1.look_at($Human2.global_transform.origin, Vector3.UP)
However, this may lead to undesired rotation where the mesh rotates up/down, so you may want to do something like this so it only rotates right/left:
var target_pos = $Human2.global_transform.origin
target_pos.y = $Human1.global_transform.origin.y
$Human1.look_at(target_pos, Vector3.UP)
That should, hopefully, make the MeshInstance nodes look at each other!
If your MeshInstance node does not face the -Z axis, then I would recommend parenting it to a Spatial node, then rotate the MeshInstance so it faces the Spatial node's negative Z axis, and then use look_at on the Spatial instead of the MeshInstance.
