"No, I meant this as a question. Can you get a down jacket on underneath your rain jacket, or rather can you get your rain jacket on over-top of a down jacket? Because I can't do that comfortably, so I take a synthetic."
Yes, you can certainly wear a shell layer over either a down or synthetic insulation layer. In your case, you wrote that you can wear your rain jacket over your synthetic jacket, but not comfortably over your down jacket. Most likely, this is because your down jacket "puffs up" more than your synthetic jacket -- and is meant for colder temps than your synthetic.
The solution is to size up a shell jacket so it can be worn alone (when cool and windy) or on top of either a synthetic or down layer (when cold and windy) -- making sure that the shell jacket is big enough so that the insulation layer underneath can "puff up" as normal. Regardless of material, if the insulation is restricted, then warmth retention will be compromised.