Yes – you can self-distribute and sell via MAS at the same time.

If you self-distribute, you need to create your own purchase validation mechanism and add it to your self-distribution app bundle (issue and check valid license keys, or subscription status check, etc.) whereas your MAS app bundle will implement Apple’s purchase validation mechanism.

Customers who have purchased via MAS will be “Apple-managed” (you’ll barely know who they are, you won’t get their emails, you won’t be able to refund them, etc.) Customers that purchased via your own system will be “incompatible” with MAS customers (as in: if someone bought via you and you provided them with a license key you issued, they won’t be able to download the app from the MAS without paying again in the MAS; by the same token people who bought via MAS won’t be able to launch your app using your self-distibution app bundle, they’ll be stuck with the MAS bundle forever)

Apple’s pricing tiers might not match your app’s price outside the MAS and could potentially be confusing for customers (plus, Apple auto-converts your app’s price tier to the customer’s local currency). Also, users will often email for support and you won’t know for sure where they bought the app (and thus, how to best support them) unless you ask them explicitly. It’s a bit of a mess and there are things you need to account for, but it’s definitely doable.