You can explicitly specifiy a file using the path and filename tag; for example:
[c:\\acufiles\\file1.dat] IDXFORMAT=17 [c:\\rmfiles\\file1.dat] IDXFORMAT=21
[C:\SITETEST\EXTFH\TEST2.dat]
you would need to enter
[C:\\SITETEST\\EXTFH\\TEST2.dat]
sets file1.dat in c:\files\acufiles to IDXFORMAT 17 and file1.dat in c:\files\rmfiles to IDXFORMAT 21.