There is no need to weld the 1st part of the couopon waith a specific process. Ther is no variable such as "Previously Used Welding Process" or "Backing Filler Metal Class" for ASME Sec IX or AWS D1.1.
The self shielded FCAW process is "Special" in the fact that the elements used to shield the puddle build up rapidly. That is one of gthe reasons the manufacturers limit the the thickness to be deposited.
Some of those elements just don't mix with other elements. FEMA is indicated to have addressed some of these issues. See this article
http://www.aws.org/wj/mar02/feature2.html .
I am not aware of D1.1 or ASME Sec IX restricting the use of mulitiple procedures on one joint or multiple processes. Obviously there is some responsibility on the part of the manufacturer to know what is prudent for fabricating. Just because the code allows something or does not, does NOT mean its a good idea.
The codes are not a replacement for people familar with welding or a good quality systems. There just books so everyone can go to the same source to argue :)