Is one backhaul better than two?

Trying to think this through makes my brain hurt.

I’ve two physical APs, X and Y, on a hill-top site each serving its own physical subnet out of sight of the other, connected by ethernet. Internet connection is through a wireless gateway 15 Km distant

I’ve the option of:

a) connecting X to the remote gateway G by backhaul and routing Y through it too, so there is a single backhaul X<>G handling all the network traffic, (a point-to-point link) or

b) giving Y its own backhaul to the same interface on G, so that G is handling two links (point-to-multipoint) but the network traffic is kept separate and divided between those links.

All gear is Mikrotik so NV2 and other RouterOS protocols are available.

What would be the ‘best’ set-up from the point of view of throughput?