  We present a method for modeling the evolution of detached double white dwarf
(DWD) binaries hosting helium donors from the end of the common envelope (CE)
phase to the onset of Roche Lobe overflow (RLOF). This is achieved by combining
detailed stellar evolution calculations of extremely low mass (ELM) helium WDs
possessing hydrogen envelopes with the the orbital shrinking of the binary
driven by gravitational radiation. We show that the consideration of hydrogen
fusion in these systems is crucial, as a significant fraction ($\approx$50%) of
future donors are expected to still be burning when mass transfer commences. We
apply our method to two detached eclipsing DWD systems, SDSS J0651+2844 and
NLTT-11748, in order to demonstrate the effect that carbon-nitrogen-oxygen
(CNO) flashes have on constraining the evolutionary history of such systems. We
find that when CNO flashes are absent on the low mass WD ($M_{2}$ < $0.18
M_{\odot}$), such as in NLTT-11748, we are able to self consistently solve for
the donor conditions at CE detachment given a reliable cooling age from the
massive WD companion. When CNO flashes occur (0.18 $M_{\odot}$ < $M_{2}$ < 0.36
$M_{\odot}$), such as in SDSS J0651+2844, the evolutionary history is
eradicated and we are unable to comment on the detachment conditions. We find
that for any donor mass our models are able to predict the conditions at
reattachment and comment on the stabilizing effects of hydrogen envelopes. This
method can be applied to a population of detached DWDs with measured donor
radii and masses.
