Spark standalone should definitely be capable of using multiple audio outs (if your sound card has them).
I am personally frustrated that those soft synths standalone apps aren't fully developed. Especially this is a case with hybrid solutions like Spark. Spark + computer + sound card should really be like independent drum machine on its own.
Standalone app cannot fully use your sound card, its sequencer cannot use its physical midi-outs, it cannot use other V-synts (which many of us probably have). Yes, I know these features would need lot of programming, but IMO it's worth it. As a performance tool, I'd really want to use it on its own.