They are always updated at the same time. When buying the Itchio version you get a Steam key, but If you buy it on Steam you don't get an Itchio key.
Another question, it says on your mini fx page "If you already have Pixel FX Designer, Juice FX or Smear FX you'll get a 100% discount when purchasing Mini FX" is there anyway for me to get a copy of this on itch since I paid for the steam versions of those programs. I can send you my receipt as well if you wish.