Mixed Rounds
Matthew, Mark and Lisa have been drinking for a little while. Luke, Eve and John arrive, who have also been drinking for some time at another establishment. The rule here can seem complex, so sit up straight and pay attention.
The two rounds must mesh together like well-oiled gears in a gearbox, or perhaps more like the two sides of a well-oiled zipper. Let's say that the status of the two rounds are like this, where the name in bold is the person due to buy the next round:
Group A: Matthew > Mark > Lisa
Group B: Luke > Eve > John
The resulting round should look like this:
Eve > Matthew > John > Mark > Luke > Lisa
The reason that Eve buys the next round, rather than Matthew, is that Matthew was already sitting down.