  The radiatively-induced splitting of masses in electroweak multiplets is
relevant for both collider phenomenology and dark matter. Precision two-loop
corrections of $\mathcal{O}$(MeV) to the triplet mass splitting in the wino
limit of the minimal supersymmetric standard model can affect particle
lifetimes by up to $40\%$. We improve on previous two-loop self-energy
calculations for the wino model by obtaining consistent input parameters to the
calculation via two-loop renormalisation-group running, and including the
effect of finite light quark masses. We also present the first two-loop
calculation of the mass splitting in an electroweak fermionic quintuplet,
corresponding to the viable form of minimal dark matter (MDM). We place
significant constraints on the lifetimes of the charged and doubly-charged
fermions in this model. We find that the two-loop mass splittings in the MDM
quintuplet are not constant in the large-mass limit, as might naively be
expected from the triplet calculation. This is due to the influence of the
additional heavy fermions in loop corrections to the gauge boson propagators.
