Whenever you try to buy a 2-player realm normally, Minecraft greets you with an error about "Maximum subscriptions".

To replicate this bug, just go into Settings, then the subscriptions tab, then click $3.99 or whatever the price is on the 2-player realm.

To get around this, do not click on the price. Click "More Info" instead that's to the right of the price tag, then click the price tag right after that.