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:
Hi!
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.