How can I make netwatch watch 2 or more hosts, and only execute a script if all specified hosts (IP's) fail?

I am pretty sure it's not possible out of the box. But I guess if I have several Netwatch entries (one per host/IP), and each of them sets a "flag", I could test that flag inside the script, and only run the script if all flags are set. How exactly would I do that?

You need the simple scheduler task, that will ping hosts that you need by a script inside task. After that, you can wright any scenarios you want. Netwatch can ping only one IP.