looks like fast track isn't multi client. I'm lucky that my RME card is, although you shouldnt have to purchase a different card.
I was able to get it to work by selecting Primary Sound Driver (DS) in sibelius. its imperative that both apps are set to same sample rate though.
Core i7 3930k - ASUS PX79 Pro - 64GB RAM - W7 Pro on 240GB Neutron SSD - RME HDSPe AIO - OCZ RevoDrive 120GB - Crucial RealSSD 256GB - 8 x HDD - Motif XS8 - Focal Solo 6 Be - Cubase 6.5 - Sibelius 6.2 - VE PRO - QL Spaces - Altiverb 6 - Hollywood Series - SC - SD2 - EWQL Pianos - MOR - Requiem Choir - VSL SE+ - Symphobia 1 and 2 - True Strike