Keston have quite a lot of info on boiler sharing on their site that I came
across when researching my own (single boiler) installation. They do a lot
of it on their commercial boilers using a plenum chamber arrangement
which looks as if it would actually short circuit the boilers' outputs, but by
clever pumping a balance is achieved. There is also a simpler arrangement
for use with the lower output Keston Celsius 25 boiler. Not your product of
course, but you may find useful info.

Can't find all the links, but there is a useful diag in this download (450k
pdf):
http://www.keston.co.uk/downloads/pdf/comm-b.pdf

Also useful, but less intuitive diagrams at:
http://www.keston.co.uk/downloads/pdf/2xcelsius-v2.pdf

plus more to search at:
http://www.keston.co.uk/downloads/index.htm

Lots of drawings under the CAD section.

HTH
--
fred