  We discuss how the double missing partner mechanism solution to the
doublet-triplet splitting problem in four-dimensional supersymmetric SU(5)
Grand Unified Theories (GUTs) can be combined with predictive models for the
quark-lepton Yukawa coupling ratios at the GUT scale. It is argued that towards
this goal a second SU(5) breaking Higgs field in the adjoint representation is
very useful and we discuss all possible renormalizable superpotentials with two
adjoint Higgs fields and calculate the constraints on the GUT scale and
effective triplet mass from a two-loop gauge coupling unification analysis. Two
explicit flavour models with different predictions for the GUT scale Yukawa
sector are presented, including shaping symmetries and a renormalizable
messenger sector. Towards calculating the rates for proton decay induced by the
exchange of colour triplets, the required Clebsch-Gordan coefficients for their
couplings are calculated for the possible dimension five and six operators.
They are provided in detailed tables in the appendix, together with additional
helpful material for GUT flavour model building.
