We've got a similar situation, only in ours the older one won't leave the younger one alone. The younger one is very, very, very laid back and quiet, so she just gets harassed and shoved around if the older one feels like it and she doesn't do anything to retaliate or stop it.
So, we divided everything up. Each horse has their own stall and corral, the corrals are separated by a common fence, so if they want to be "together" they can both go over to the fence, or go away from it when they've had enough. Did the same with the pastures for turnout, 2 pastures with a common fence.
Now we do let them be together, just not when both are in heat (both are mares) and not for extended periods of time. So we'll put both of them in the 3rd pasture that's completely separate from everything else for 3-4 hours then separate them so each one has their own pasture for the rest of the afternoon.
Separating them has other benefits as well. We designed the property so each corral has a gate into a pasture so we can leave the gate open and each horse can choose between pasture, corral and stall during the day. Also means that in winter we only need one heated water trough, it's located in the common fencing of the corral, so both horses have water in their corral and during turnout time, they just go into their corral for water.
They do enjoy being together, had a kid leave the gate in between the 2 corrals open and the sillies jammed themselves into the same stall but both horses are happy with some together time and some apart time.