Many quite easy ways to do it. I have seen how some other railroads did it. The Q's rotary stationed in Alliance had 2 aux cars. I have pictures of both cars but have never seen them actually attached to the rotary.Did see something that said they pumped water from the aux into the actual tender. You can even see the plow side of things on the OM and OY and how the plumbing works on the aux water cars 0471 and 0472 which are ex tank cars. The ex tender aux car was gone and 0471 already assigned to the OM when OM received its current tender.
I would sure like to see how the D&RGW plumbed their aux water cars for the rotaries. The ex tender aux on OY and OM ran backwards so the water hatches on the tender and the aux were close together. Been told to make filling easier. So I'm sure they just had a simple pipe running under the car to the water legs. But I would like to see it for modeling purposes. The piping runs under the right tender side sill on OY and on OM's current and pre 1962 tender. The piping has the same connections as a normal tender. It appears that they did not refill the actual tender with water from the aux, but maybe pulled the right side fill from the aux and the left from the actual tender.
The RGS rotary has the same piping arrangement with same fittings, The RGS aux water cars for the rotary were basically unchanged and retained the normal locomotive connections at the water legs as the tenders ran in the correct direction.
Maybe Earl can chime in if he ever used the aux (ex tank car, OM's 0471) while plowing with the OY. I know it was along on some of the trips but I don't know if it was used.