I looked at your component and I vaguely understand the potential, but just doublechecking here.

What I need:
1 joomla installation, 1 DB
2 domainnames attached to that 1 joomla installation/DB

Behaviour:
Say you got a menu 'mainmenu', and it consists of subitems Home, Products, Forum. The primary domainname is 'domain1.com'
On click 'Forum' the 'domain1.com' shoudl change into 'domain2.com'. The user gets the idea that he's on a 'different' website, however it still uses the same joomla installation.
Additionally: all items listed 'under' 'Forum' (which should have the 'domain2.com' should all be 'automatically' use the 'domain2.com'

So in short:
MainMenu -> domain1.com
Products -> domain1.com
++ProdcutsSub1 -> domain1.com
++ProductsSub2 -> domain1.com
Forum -> domain2.com
++ForumSub2 -> domain2.com
(all subitems of 'Forum' should use domain2.com 'automatically')

(easily) Possible, or did I misinterpret the description?

JMS does not share the content neither the users.
So if you create a menu to switch form a website to another one, the user info wiil be lost.
You must consider each site as totally independent.
JMS only share the files and folder on the disk.
See the FAQ that explain all that in detail.