Do the two sites have their own equipment as well as "shared" rental equipment? If so, you should consider not trying to share the rental equipment between two sites. Create a third site. Put the shared rental equipment in that. Treat it as its own site. It has inventory of equipment for rent. The two sites then reference that single common site for the shared inventory.
Otherwise, your inventory management will become quickly a nightmare of frustration. You will have two companies, each having private inventory but then trying to update each others for the shard data.
It the two sites have exactly the same equipment and you just have two names for the one company then it is simply a matter of creating two websites with the same database. But, that is a rare condition.