Hello John,
If you want to use Hauptwerk (or any other virtual instrument software) with a VST/AU sequencer application (such as Cubase, Logic, Sibelius, etc.) on Windows then you would normally have to have the audio output from the virtual instrument being played through the sequencer, due to the limitations of the ASIO architecture that you mention (i.e. that ASIO only allows one driver to be used at a time, and only by one application at a time).
For generic VST/AU sequencers like Cubase, Logic, Sonar and Reaper that isn't an issue for multi-channel output because the Hauptwerk VST Link is designed to support 32 audio channels (16 stereo pairs) and those VST/AU sequencers support multiple audio outputs from VST/AU instruments, so you can use multi-channel audio output fully.
If Sibelius supports multi-channel audio output from a VST plug-in then (e.g. via its mixer) then that's fine too (I don't know off-hand). However, if Sibelius was limited to a single stereo output from a VST plug-in then that would be a limitation of Sibelius which effectively would just have to be lived with.
On Apple Macs you have more options since OS X has high-performance audio built into it (Core Audio - at least as good as ASIO), and Core Audio doesn't have the single-driver/application limitations of ASIO, and Macs have a built-in audio output that is capable of being used as a reliable low-latency audio clock, so you could potentially use your main audio interface for direct multi-channel audio output from Hauptwerk (without routing audio via VST) and just configure Sibelius to use the built-in audio output, which will give good timing and performance from both applications.
If you want to use Hauptwerk (or any other virtual instrument software) with a VST/AU sequencer application (such as Cubase, Logic, Sibelius, etc.) on Windows then you would normally have to have the audio output from the virtual instrument being played through the sequencer, due to the limitations of the ASIO architecture that you mention (i.e. that ASIO only allows one driver to be used at a time, and only by one application at a time).
For generic VST/AU sequencers like Cubase, Logic, Sonar and Reaper that isn't an issue for multi-channel output because the Hauptwerk VST Link is designed to support 32 audio channels (16 stereo pairs) and those VST/AU sequencers support multiple audio outputs from VST/AU instruments, so you can use multi-channel audio output fully.
If Sibelius supports multi-channel audio output from a VST plug-in then (e.g. via its mixer) then that's fine too (I don't know off-hand). However, if Sibelius was limited to a single stereo output from a VST plug-in then that would be a limitation of Sibelius which effectively would just have to be lived with.
On Apple Macs you have more options since OS X has high-performance audio built into it (Core Audio - at least as good as ASIO), and Core Audio doesn't have the single-driver/application limitations of ASIO, and Macs have a built-in audio output that is capable of being used as a reliable low-latency audio clock, so you could potentially use your main audio interface for direct multi-channel audio output from Hauptwerk (without routing audio via VST) and just configure Sibelius to use the built-in audio output, which will give good timing and performance from both applications.
Best regards, Martin.
Hauptwerk software designer/developer, Milan Digital Audio.
Hauptwerk software designer/developer, Milan Digital Audio.