I am wondering if it is possible to have multiple pagetrees under root, without changing to a multisite license.
What are the pros and cons of structuring a site like this?
The reason is to separate extranet and intranet.
You can have multiple page trees under the root, as that is not what determines if the solution is multi-site. I've done this a couple time, for example if I want to have a separate content "bucket" outside of the main site page tree. It's not done very often, but it's certainly possible to do.
It becomes multi-site when you have multiple websites configured in Admin mode, usually when you have multiple hostnames, and each site points to a different start page (usually each start page is a child of the root).
Is your extranet and intranet on different hostnames?
That is not decided yet. Could we have different hostnames without a multisite license?
Yes. You can point multiple hostnames to the site, but they would all be pointed to the same start page: