This paper explores working methods and instrument design for musical performance sessions (studio and live) where cross-adaptive techniques for audio processing are utilized. Cross-adaptive processing uses feature extraction methods and digital processing to allow the actions of one acoustic instrument to influence the timbre of another. Even though the physical interface for the musician is the familiar acoustic instrument, the musical dimensions controlled with the actions on the instrument have been expanded radically. For this reason, and when used in live performance, the cross-adaptive methods constitute new interfaces for musical expression. Not only do the musician control his or her own instrumental expression, but the instrumental actions directly influence the timbre of another instrument in the ensemble, while their own instrument’s sound is modified by the actions of other musicians. In the present paper we illustrate and discuss some design issues relating to the configuration and composition of such tools for different musical situations. Such configurations include among other things the mapping of modulators, the choice of applied effects and processing methods.