Musical duets are a type of creative partnership with a long history of artistic practice. What can they tell us about creative partnerships between a human and a computer? To explore this question, we implemented an activity-based model of duet interaction in software designed to support musical metacreation and investigated the experience of performing with it. The activity-based model allowed for the application of reflexive interactive processes, previously used in dialogic interaction, to a synchronous musical performance context. The experience of improvising with the computational agent was evaluated by expert musicians, who reported that interactions were fun, engaging, and challenging, despite some obvious limitations in the musical sophistication of the software. These findings reinforce the idea that even simple metacreative systems can stimulate creative partnerships and, further, that creative human-machine duet partnerships may well produce, like human-human duet partnerships, more than the sum of their parts.