Problem:
you want 2 diffrent forum with 1 login

Solution:
YAF is just a control so call it using the boardid as a parameter. For
instance, if you look at the default.aspx it has code that looks like
this:

<form runat="server" enctype="multipart/form-data">
<yaf:forum runat="server"/>
</form>

Now
go into admin and create a new board (Boards) and make note of the ID.
Make another file called "secondboard.aspx" (or whatever) and change
call to YAF to include the boardid #:

<form runat="server" enctype="multipart/form-data">
<yaf:forum boardid="2" runat="server"/>
</form>

(Boardid
is whatever you want… you could have it reference a million different
boards if you create them — categoryid is also supported.)