I wasn't aware that you could go long distances with the signal (EIB bus).
If i may believe following document: http://www.merten.de/download/DL_ti_gb/5_instabus.pdf, i should easily bridge my 2 installations (about 60m from each other).
If this is the case, linuxmce will work out of the box...
I would preface my comments by saying that I haven't done it, but my understanding is that this is correct. You might be wise to seek advice (from a knx retailer or training centre - I know that www.knxshop.co.uk
would do so in the UK). I have done a short knx course, and I understand this to be an integral part of the design philosophy of knx. You could certainly arrange the buildings using line couplers or bus couplers (I don't know which is appropriate, as I have not done this, and it is also dependent on the requirements of the individual buildings) in such a way that conversion to independent systems (at some time in the future) was relatively trivial.
You might want to ask about the appropriate power supply/supplies and their location, depending on the size of the installation and subsequent load.
It should be doable, though, whatever the situation (if you have the budget available....). KNX is used extensively in the Heathrow Terminal 5 building, and that's quite big (and KNX was not in any way responsible with the teething trouble after commissioning of that building).
KNX is a good fit for large projects. Companies like ABB have systems teams that design, install, and commission their own large scale building management systems. They don't call it as such, but it's KNX. It's only recently starting to drift down to the residential and small business level.