I did a search on this and found a start, posted by Richard:

to have 2 forums share a database, you just need to change 2 lines in the config.asp file of the second forum:

forum #1's config.asp:

strTablePrefix = "FORUM_"
strMemberTablePrefix = "FORUM_"

forum #2's config.asp:

strTablePrefix = "FORUM2_"
strMemberTablePrefix = "FORUM2_"

Doing it this way, you can use the same database for both forums. No data will be shared between each of them.
If you want to share the Members table then:

forum #2's config.asp:

strTablePrefix = "FORUM2_"
strMemberTablePrefix = "FORUM_"

I applied this, only to bump into this:

Microsoft JET Database Engine error '80040e37'

The Microsoft Jet database engine cannot find the input table or query 'FORUM2_PM'. Make sure it exists and that its name is spelled correctly.
inc_header.asp, line 183


So, basically the forum is checking for the Forum2_PM field,
which doesn't exist. I want things like Users, PM's, amount of posts,
points and active users all loaded from Forum 1's database, then
have things as categories and in-forumposts load from Forum2 fields.
Anyway I can trick the forum/database into believing that some of the
Forum2 fields are Forum_ fields? So, in that way you can send a PM
in forum1, which will also be readable in Forum2, 3, 4, etc and with
that turning a one-time registration into some sort of "Forum Passport" ?
Any help would be appreciated cause I expect to bump into this
error a couple of times with different fields, so any solution is
welcome, maybe a Config2.asp file in which fields are specified to
be shared fields or forum only fields?
*help*

Thanks in advance!

D<