More on the tandem than at other times (maybe because we are faster than individually, and this is frequently underestimated by drivers
), I, or my lovely stoker, have used an outstretched palm to warn drivers not to overtake until we can see the way to be clear.
I find that a stoker doing 'jazz hands', or putting her hands on her head, gets a message across effectively. The observant among you will see that Superstoker pedalled most of Freewheel with his hand behind his back, a practice he uses at other times as well.