ok dim pro + SD 3 + true piano are in sonar... but you treat the input from rewire as the trigger / to send the notes..
I've not used Sibelius , but assume it supports VST's ? you can use Dim Pro and SD3 in it, BUT not True pianos perhaps as it checks it's running in a only sonar
there is a usergroup on yahoo and a facebook page for the users of it, I'd say speak to them..
rewire, you have master / slave , where 1 app is the main one and the 2nd syncs to it.. think of it as simon says but for audio / midi.. thats the very easy version I can say
you might wanna speak to folk on "www.sibeliususers.org" or their facebook page
but it's not something I'd buy, due to it not being 100% due to it's future