JACK is an open source audio app that allows the routing of audio channels and MIDI ports between different applications. You can freely connect them in any way you want. Besides this it also offers other ways to allow your audio and sequencer apps to communicate, for example record/playback synchronization between DAWs and sequencer apps. Arbitrary data sharing among apps and much more.
I have used it quite extensively in its first iteration routing audio between DAW's, over networks and even over the internet and around the country. Although its relatively new to iOS according to Schoenebeck it provides "the most professional and powerful environment for your audio apps, highly optimized with explicit multi core support, low latency and maturity which it gained over many years of usage and development on other operating systems already."
Tyler walks us through their flagship trigger sequencer