John,
What version of GEMPAK are you running?
The gdfile line length is 128 characters, so if you
exceed that, you would be out of luck. Then I would suggest making
symbolic links from the full names to filea and fileb like you
outlined.
Another way would be to use the template feature such that you could say
gdfile = ruc2|yymmdd/hhnn+ruc2|yymmdd/hhmm
Any and all should work (comparing against the current version of 5.7.2)
I used (excerpted for relevant parameters here):
GDFILE = ruc2|040609/1800+ruc2|040609/1900
GDATTIM = f000
GLEVEL = 500
GVCORD = pres
GDPFUN = hght^040609/1900f000+2 ! hght^040609/1800f000+1
TYPE = c
CINT = 60
LINE = 6 ! 5
TITLE = 1/-1 ! 1/-2
The above plots the 500mb hghts using the file+1 and file+2 which you
would need if you wanted to use fields from either in a computation
such as:
GDPFUN = sub(hght^040609/1900f000+2,hght^040609/1800f000+1)
You can also suceed with the following if you are not requesting
fields from both files in one computation:
:
GDFILE = ruc2|040609/1800 ! ruc2|040609/1900
GDPFUN = hght ! hght
Steve Chiswell
Unidata User Support