With 1.5mm walls, you MIGHT get 2 shells but more likely you will get 1 shell and a single line of material between to make up the difference (.4mm nozzle - 1 shell = .8mm total width 1.5-.8=.7 .7 is "too small" for an additional shell. 1 shell and 100% fill in that area is likely your best bet.

Regardless of how you print it though, 1.5mm thickness is not much for an FDM part. It's just not going to be terribly strong unless it's short. I've managed 2mm walls that were actually quite sturdy but they were very short.