Hi Andi,
I am using Sibelius 5.2.5
I seem to have solved it, although the method I have used in somewhat unexpected, the details follow as it may help other users.
The problem I had was that the viola has a lower range in "legato" than it does in "sustain". Following a legato passage, Viola 2 was not resetting to it's default sustain playback, but Viola 1 was (hence Viola 1 was able to perform the high range, but Viola 2 was not).
I was able to solve it from within the Sibelius mixer.
Eg, for Viola 1, in the drop-down menu on the third line, I selected the first instance of the vienna.viola.ensemble. The instrument then defaulted to MIDI channel 5.
Then for Viola 2, in the drop-down menu on the third line, I selected the second instance of the vienna.viola.ensemble. This then also defaulted to MIDI channel 5.
Previously I was changing the second Viola to MIDI channel 6. But if it is left to MIDI channel 5, as unexpected as it seems, both Violas play back completely independantly (as if they are on different MIDI channels), and both reset to sustained. Both are routed independantly on the Ensemble mixer too.
It's very strange, but it works. I have also checked this with the 4 violin ensembles, all are defaulting on MIDI channel 1, but as long as they use different vienna.violin.ensemble options from the drop down menu on line 3 of Sibelius' mixer, they remain completely independant.
Everything seems ok and sounds great.
Many thanks,
Shayne