So your blanket will only output at the fixed panel’s specs if connected in parallel.
Very unusual if panels these days don’t have inbuilt diodes to stop feedback from the battery.
Are you referring to Bypass diodes perhaps? These are on every panel and designed to bypass a panel when they are in series but shaded, the usual configuration for houses.
I am referring to Blocking diodes, I have never seen these inbuilt. These would have to be added to stop shaded parallel panels from acting as a load and possibly overheating. They were used in the old days or when you directly connect a panel to the battery without a regulator. That hardly every happens these days. But they are also used when panels are in parallel for the same reason - to stop the panels using power.
My advice is to put the panels in parallel but includes schotley diodes in series with each panel. You can get 20A ones at Jaycar for about $3 each. I would not recommend putting panels in parallel without the additional blocking diodes.