1. Open a.cpp in one side
2. Open b.cpp in other side
3. When b.cpp side has focus, click a.cpp side so that a.cpp side has focus. Then press F4 as soon as possible.
4. You will see b.cpp side takes focus again and open b.hpp in b.cpp side.

If you take some time before press F4 after click a.cpp side, it works as expected.