They WOULD fit; just not perfectly -- how important is that to you?
THEORETICALLY, the 1.5 tube in a 2.125 tire would stretch thin, making it easier to go flat; but since a 26x2.0 tube, inflated outside the tire, is as big as a hula hoop (4' across, tube 3-4" thick), I'm not too worried about that.
The only problems with the bigger tube in the smaller tire is a bigger chance of pinching the tube in the bead when mounting, and an ounce or two more rubber.