So now I got 2 big expansions, and I don't fully understand how the bonuses about -1 investigator advantage when using 2 board expansions. Can someone explain it to me?

Sure! If you're playing with X investigators, you pretend as if you're playing with X-1 investigators for the purposes of counting gate limit, monster limit, outskirts limit, etc.

The only thing unaffected is the number of successes to remove a doom token during final combat.

So if you're playing with 5 investigators with no expansion boards or one expansion board, normally you will have these settings:

• Monster limit: 8
• Outskirts limit: 3
• Simultaneous gates to awaken AO: 6
• Monsters per newly opened gate: 2
• +5 stamina on the "Riots" rumor per round
• Gate trophies required to win by "close all gates" condition: 5

But, if you're using two expansion boards with 5 investigators, your new settings will be this:

• Monster limit: 7
• Outskirts limit: 4
• Simultaneous gates to awaken AO: 7*
• Monsters per newly opened gate: 1
• +4 stamina on the "Riots" rumor per round
• Gate trophies required to win by "close all gates" condition: 4

*If your two boards happen to be Dunwich and Innsmouth, this gets an additional +1!

So you play as though you're using only 4 investigators. The differences may not look like a lot, but they add up. And in particular, the "gate trophies to win by closing all gates" can be really helpful under certain circumstances (such as Atlach-Nacha!).

Out of interest, may I ask which two expansion boards you're using?

Well, I always had Dunwich, and I just got Kingsport (want to try Tony Morgan, plus the Epic Battle cards will add a great deal to my games)

Thanks for the clarification.