Can I add another path to the current search directory?
I can call another script with source("file1.R") and another script in another "sibling" folder with source("..\\otherdir\\file2.R).
I think of something like "addwd("..\\otherdir\\")" to call file2.R with source("file2.R").
I don't think this would lead to too much ambiguity since the working directory is always chosen first.
Not sure I completely follow what you are trying to do. However, if you're trying to temporarily change the working directory to the folder where file2.R is located before it is sourced, you can use the chdir = TRUE argument.