Which atoms are orbited by shared electron?

From a quantum mechanical point of view, an electron is described by means of a wavefunction $psi(textbf{x})$ rather than by an orbital motion around some atom nucleus. The absolute square of the wavefunction $mid{psi(textbf{x})}mid^{2}$ provides a probability distribution for where the given electron may be found to be when performing a position measurement. The way you reconcile this concepts with your knowledge from school physics is to associate to each of the orbitals along which one was told electrons go along to such a wavefunction, which results in a probability distribution “peaked” in space around the corresponding orbital trajectory.

The wavefunction of an electron involved in a covalent bond provides a probability distribution $mid{psi(textbf{x})}mid^{2}$ that is smeared out betweeen the two atoms “sharing” the electron.