One or multiple site IDs

We are setting up Matomo for analytics and wondering what the best practices are for cross sub-domain tracking. Our setup is a landing page (a.foobar.com) and the actual app (b.foobar.com).

Would it be better to set them up as separate sites, and if so, is there an easy way to link for Funnel tracking? Or is it okay to use the same site id?

Thanks!