This is a big technical challenge I think. And just as mentioned above, it contradicts the idea behind contactless payments: simplicity, secure and speed.
But, I also understand your needs. It would help is you can reassign you T&P or card 💳 very fast to a different account. Let's say by pressing one single button (shortcut for instance).. Right now it takes several clicks. That way you can change accounts almost instantly when you are in line.