Less is always more, especially early on in a project.
I've learned to never split code bases until it's absolutely apparent that it must be done.
Making 2 apps means twice as much work in so many ways. Two bundle exec rails server
to run, two bundle exec rspec
to run, cd ..
, cd web_app1
, cd web_app2
over and over.
You also have to deploy two apps.
So, make one app. Split it up later, if and when you need to.
If they are on the same subdomain, you can share cookies, and easily achieve SSO.
And there is no reason you can't use something like nginx to put them behind their entirely separate domains, as a deployment consideration.