The Hindenburg medallions overprinted Luxemburg lost their validity at the end of 1942; however, the unoverprinted Hindenburgs remained valid in the Reich until May 1945. Thus, the 25 Rpf. Hindenburg is properly used with the 60 Rpf. Hitler-Head on this incoming parcel card from Tiefenbach über Oberstdorf in Bavaria, 4 February 1943, to Esch/Alzig, 9 February 1943.
Similar Hindenburg & Hitler-Head mixed frankings, therefore, should have been possible from Luxembourg, as well, presumably from April 1, 1941, when all German stamps became valid for use in Luxembourg, but I have yet to see an example. Have you?
The World War II parcel cards invite study by specialists. Almost nothing has been written about them. The code numbers on the right side of the parcel label come from a grid book used to calculate distance and rate. Here the charge was 60 Rpf. for 5 kg. of weight to Esch/Alzig plus 10 Rpf. for 200 RM of insured value and 15 Rpf. for home delivery. As shown on the reverse, the home delivery fee was refunded when the parcel was picked up at the post office.