Same subnet over VPN

[Jarhead]

So I'm contemplating a redundant server between 2 buildings and looking for some info beforehand.
Here's what I need to do:
I'll setup a Xenserver in each building and use everRun for VM redundancy.
To do this I need to have the remote site use an IP from the local subnet. I have an OpenVPN between building already but it's a tun VPN so no layer2.

My questions are:
Does anyone know how to have the remote site use a local subnet?
I'm assuming a tap VPN might do this.
Any other way?

If a tap VPN works, how would that play with the existing tun VPN between these sites?
My thinking is keep the existing VPN and add a tap just for the server.

Any thoughts or other ways to do this?