You'll be asked to use the app that matches the brand of card you're paying with. For example, if you're paying with a Halifax card, you'll be prompted to use the Halifax app. If you're paying with a Lloyds card, you'll be prompted to use the Lloyds app.
Once you’ve moved to the Lloyds app, all future payments you make with your Halifax card will be approved in the Lloyds app.