Answer:
No.
Both tubesheets must have the same material, this is a design codes requirement. That's why in the Tubesheet Dialog there is only one material tab the material of the tubesheet, the reference is always the material of the fixed/stationary tubesheet. This will avoid any errors in the calculations.
However, while using AutoPIPE Vessel 46.00.00.165 or lower, seems that a hidden reference is not updated when material of the tubesheet is changed, this explains why an unused material appears in the Change Material Dialog and also in the drawing's BOM. This has been logged (1664012) and will be fixed in a future version of the program. The material of the floating tubesheet should be automatically updated throughout the program, as well as in any reports or drawings, whenever a change is made to the material on the tubesheet dialog.