  Precise predictions for Higgs boson masses in the Minimal Supersymmetric
Standard Model can be obtained by combining fixed-order calculations with
effective field theory (EFT) methods for the resummation of large logarithms in
case of heavy superpartners. This hybrid approach is implemented in the
computer code FeynHiggs and has been applied in previous studies for
calculating the mass of the lightest CP-even Higgs boson for low, intermediate
and high SUSY scales. In these works it was assumed that the non-standard Higgs
bosons share a common mass scale with the supersymmetric squark particles,
leaving the Standard Model as the low-energy EFT. In this article, we relax
this restriction and report on the implemention of a Two-Higgs-Doublet Model
(THDM) as effective theory below the SUSY scale into our hybrid approach. We
explain in detail the various steps how our EFT calculation is consistently
combined with the fixed-order calculation within the code FeynHiggs. In our
numerical investigation we find effects on the mass of the lightest CP-even
Higgs boson h of up to 9 GeV in scenarios with low $M_A$ , low $\tan\beta$ and
high SUSY scales, when compared with previous versions of FeynHiggs.
Comparisons to other publicly available pure EFT codes with a THDM show good
agreement. Effects on the mass of the second lightest CP-even Higgs boson H are
found to be negligible in the phenomenologically interesting parameter regions
where H can be traded for h as the experimentally observed Higgs particle.
