This feature is only available in SoundPrism Pro - get it here:
With SoundPrism Pro it is possible to control synthesizer apps that are running in the background on the same device. One of the many apps that officially supports this technology is TempoRubato's NlogSynth Pro for iPad and Nlog MIDI Synth for iPhone and iPod Touch.
Many more compatible synthesizer apps are available for the iPad and for the iPhone and iPod touch.
Setting up SoundPrism Pro to work together with NLog is incredibly easy. Just start your iOS device, then close any apps that are running in the background as shown in the video below. After that all you need to do is start NLog and SoundPrism Pro and and optionally disable the internal sound engine of SoundPrism
NLog also support MIDI recording and lets you email the files to your desktop Mac or PC after you're done. For details please refer to the Nlog manuals.
Of course it's possible to control multiple apps at the same time with SoundPrism Pro. Here's a video demonstrating how such a setup sounds like and how to configure it.