  We investigate the formation of the ten double-lined double white dwarfs that
have been observed so far. A detailed stellar evolution code is used to
calculate grids of single-star and binary models and we use these to
reconstruct possible evolutionary scenarios. We apply various criteria to
select the acceptable solutions from these scenarios. We confirm the conclusion
of Nelemans et al. (2000) that formation via conservative mass transfer and a
common envelope with spiral-in based on energy balance or via two such
spiralins cannot explain the formation of all observed systems. We investigate
three different prescriptions of envelope ejection due to dynamical mass loss
with angular-momentum balance and show that they can explain the observed
masses and orbital periods well. Next, we demand that the age difference of our
model is comparable to the observed cooling-age difference and show that this
puts a strong constraint on the model solutions. However, the scenario in which
the primary loses its envelope in an isotropic wind and the secondary transfers
its envelope, which is then re-emitted isotropically, can explain the observed
age differences as well. One of these solutions explains the DB-nature of the
oldest white dwarf in PG1115+116 along the evolutionary scenario proposed by
Maxted et al. (2002a), in which the helium core of the primary becomes exposed
due to envelope ejection, evolves into a giant phase and loses its
hydrogen-rich outer layers.
