I'd also suggest that you not worry about them. Drafting you is their choice, and when you're not smooth due to fatigue, it's their responsibility to pick up the warning signs and either pass or gamble with staying on your wheel.

However, you're certainly entitled to an enjoyable ride, and if people drafting you gives you stress, you should say something.

I think this would be one of those cases that white lies are okay. Just tell them you're new to the tandem and them drafting you is stressing you out. Or tell them your stoker doesn't like it. Whatever makes them understand that you're not being selfish or competitive, but them being back there is messing up your day. If they're at all considerate and understand your goal of a stress-free, enjoyable ride, they'd pass. If they don't, then they're a bung-hole and you definitely can stop worrying about their well-being.