  The origin of Uranus and Neptune has long been challenging to explain, due to
the large orbital distances from the Sun. After a planetary embryo has been
formed, the main accretion processes are likely pebble, gas and planetesimal
accretion. Previous studies of Uranus and Neptune formation typically don't
consider all three processes; and furthermore, do not investigate how the
formation of the outer planet impacts the inner planet. In this paper we study
the concurrent formation of Uranus and Neptune via both pebble, gas and
planetesimal accretion. We use a dust-evolution model to predict the size and
mass flux of pebbles, and derive our own fit for gas accretion. We do not
include migration, but consider a wide range of formation locations between 12
and 40au. If the planetary embryos form at the same time and with the same
mass, our formation model with an evolving dust population is unable to produce
Uranus and Neptune analogues. This is because the mass difference between the
planets and the H-He mass fractions become too high. However, if the outer
planetary embryo forms earlier and/or more massive than the inner embryo, the
two planets do form in a few instances when the disk is metal-rich and
dissipates after a few Myr. Furthermore, our study suggests that in-situ
formation is rather unlikely. Nethertheless, giant impacts and/or migration
could potentially aid in the formation, and future studies including these
processes could bring us one step closer to understanding how Uranus and
Neptune formed.
