https://techcrunch.com/2020/05/15/how-will-europes-coronavirus-contacts-tracing-apps-work-across-borders/

A major question mark attached to national coronavirus contacts tracing apps is whether they will function when citizens of one country travel to another. Or will people be asked to download and use multiple apps if they’re traveling across borders?

Having to use multiple apps when travelling would further complicate an unproven technology which seeks to repurpose standard smartphone components for estimating viral exposure — a task for which our mobile devices were never intended.