Differin (adapalene 0.1%) and Skinoren (Azelaic acid 20%)

Can you apply Differin gel then Skinoren cream at night?

Can you apply either Differin or Skinoren cream on top of a serum? (Eg. serum, then Differin gel) or will it reduce its effectiveness? Some people say apply to clean dry skin, others say it’s fine or even better to apply after a serum.