display two variables at same time from two diferente files

[Isilda da Cunha Menezes]

Hello!
I want to create a binary file where do I display:

d (prec-prec (t-1)) + (acon-acon (t-1))

but my variable prec belongs to one data file and the variable acon
belongs to another data file that  I open.

I opened the first data file who have acon, and define
defines aa = acon
then open the other data file who have prec.

when I display the data I write

set dfile  2

d (prec-prec (t-1)) + (aa-aa (t-1))

but in my output data I only have data from prec,  don't makes the
calculation.

I try to put dfile 1 2, but I get a error.

Can help me?

Best regards
Isilda