Probably worth posting this on the MD110 forum rather than general.
I had a 3 site network with a shared extension number range using DPNSS links between 2 MD110's and avaya system. So it is doable.
You can set the extensions for a site up as external destinations pointing to the appropriate route for the other site. The only problem with this is you end up with a very scrappy extension and external destination table. Also if they move and take numbers from site to site you have to then re-prog the routing on both switches.
The alternative is to set up a route code for each site. So you have 9 for an outside line and another digit for the inter-site link. This works quite well and is far less hassle when people move. It also allows you to have the same number existing on both sites.
The only problem with the 2nd method comes depending on how the calls are delivered. If all inbound traffic comes into one site and needs re-routing through the link then you can't have a duplicate range and will need to set up each extension as an external destination again.
I advise splitting the number range, half on each site and set up an external destination number range and don't let them move numbers between sites.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
umm,it IS supposed to do that, right??