If the lane is wide enough for lane sharing then I'd expect the cyclists to be aware of traffic approaching and the inside rider will pull behind the outside one when they see an approaching vehicle (perhaps waiting for a sign they have been noticed). It should be timed such that the approaching driver would not need to considerably slow down. If the riders have waited until the approaching driver is at their speed they have already failed at being considerate.
However in this particular case there is little traffic and most importantly the OP indicated that the driver had already started overtaking. So the cyclists should do nothing but hold their line side by side as the overtaking driver will be soon past them.