Haha, the answers are exactly what I was looking for 🙂
However, it seems to me that it is really different from what the OP was asking for...
I think that most likely what you wished for is this : | multisearch [search index=Obs1 | eval alias= "A"] [search index=* sourcetype =OBS2 | eval alias="B"] | eval mynormalizesidField=(If(alias=="A",ID,ID))
Basically, you run two differents search, mark in one field which corresponds to which and then do your formating (which you could easily conduct in the respective searches btw 😉 )
I guess the answer is quite late though... But might be upvoted if someone having the same problem as you find this post (quite unlinkely IMHO, since I guess the question is kind of named in a measleading way...)
I do not dare upvote the answers that helped me since objectively I do not think it answers the OP, but still thank you !