rok, no doubt frogman can give a more complete answer. But I'll share a recent experience.
Friday evening I attended a concert by The Academy of Saint Martin In The Fields (a top level chamber orchestra for those who don't listen to classical music). The famed violinist Joshua Bell is now music director, replacing Sir Neville Marriner. Bell performed as first violinist and did in fact conduct from his chair there. Not the usual direction through each piece as a traditional conductor would do but directing the downbeat and stops with his bow. If other directions were existent perhaps frog can suggest those.
Whether duo, trio, quartet, etc. up to chamber orchestras (maybe 30 some members) someone must be designated to give minimal directions, even if it is simply a nod of the head.