There are myriad cases where this is not true, the details of how these tax credits are structured can lead to significantly adverse outcomes. It is quite possible to pay more taxes in aggregate than either country separately.
It is certainly possible, but also implies a complex mix of different income types and very specific national pairing.
The usual case where there is a tax treaty is that the expat pays taxes at the rate of the country in which they work. It's really that simple for most people.
