If you have 2 forums under the same domain then with a replicated user base then you'd need to set the cookie path to "/" so it will be available to the entire domain.

Another option to look at is the category seperation feature. If you don't have a lot of forums this might be an option. One set of forums can be linked to ubbthreads.php?Cat=1,2,3 and the second would be ubbthreads.php?Cat=4,5,6 The numbers would be changed to correspond to your actual category numbers but this only displays forums under those certain categories.

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Rick Baker
UBBThreads developer