Can two apps share the same customer list?

Not today. A record lives inside exactly one app, and a link field can only point at collections in that same app. There is no way to point a link at another app, and no search that runs across apps.

What to do instead: keep one app with several collections in it - a Customers collection beside the rest of your work - and link between those collections freely. This is the arrangement that gives you one shared customer list.

If the two apps really must stay separate: you can export from one app and import into the other (export the app, then import the CSV or Excel file). Be clear about the cost before you rely on it: that is a copy, and it does not stay in sync. Nothing reconciles the two lists afterwards, so a change in one will not appear in the other.