Type "interference effect" into PubMed and you'll turn up several studies that have found that attempting to simultaneously train for both strength and endurance can be detrimental to the development of either. Not all studies have observed this, however, which has led to the
hypothesis that it depends on the overall training load. IOW, it seems
likely that an athlete attempting to
maximize their endurance cycling performance will not be able to do so if they simultaneously perform resistance training. This conclusion
is consistent with data showing that the muscular adaptations to resistance training - in particular, hypertrophy leading to a reduction in capillary and mitochondrial density - are antithetical to endurance.
(And if you don't like that answer, how about this one: where's the data
supporting the claim that resistance training improves the performance of endurance cyclists?
)