Atomic transition involving two electrons In the Helium-like Iron ion, Fe XXV, there is a transition from $1s2p$ to $2s^2$, and the energy of the two levels are measured as 6667.5686 eV and 13546.26 eV.  It seems like this transition involves jumps of two electrons, so will we simply observe a single emission/absorption line for this transition or two lines for each electron transition?
 A: The number of absorption or emission lines depends on the number of photons involved in the process, not the number of electrons. There could very well be a case where a single photon transitions between two different states that involve changes in quantum numbers for two electrons. In an atom, the electrons are often tightly coupled, so it helps to think of transitions between eigenstates of the multiple-electron wavefunction, not of transitions between single electron states.
I would be interested to know what your source is for the transitions and energy levels. Usually one transition is a process with one photon. Transitions can occur one after another, causing emission of multiple photons, but that counts as multiple transitions. Second order processes exist that require two photons present at once, but the spectral signature of those is a dim, diffuse line rather than two distinct lines.
In terms of angular momentum, $1s2p$ has $l_1 = 0, l_2 = 1$, and $2s^2$ has $l_1 = 0, l_2 = 0$, so it looks consistent with a transition involving a single photon.
EDIT: I took a look at the NIST page you linked, and yes, each line in the table corresponds to a single spectral line. The column headed "Ritz Wavelength Vac" is the wavelength of the photon. Each line is the result of experimental data, and if you want to see the paper with the source for the measurement, just click the links to the right.
