Clinical effects often are not linearly related to medication dose. This is because most psychotropic medications act at multiple CNS receptors, with differing affinities.
For example, trazodone and mirtazapine are both antidepressants with sedating properties that saturate at relatively low doses. This is because the medications have high affinities for histamine receptors, which are responsible for the sedating effects. At higher doses, receptors for which the drugs have lower overall affinities start to be affected. In particular, dopamine and norepinephrine receptor activation can have activating effects that override the sedating effects of the saturated histamine receptors, resulting in less sedation at higher doses. See
For example, see below for Table 3 from the second reference above, listing receptor occupancy rates at different doses of trazodone. Histamine H1 receptors are already 84% blockaded at the 50 mg dose. Dopamine and norepi transporters are minimally saturated at this dose (10% and 14%), where as by 150 mg they are 25 and 33% saturated respectively, and would go higher at more depression-relevant doses like 200-300 mg/d (not included in this table).
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This is why these medications are used at low doses for sleep (7.5 mg for mirtazapine and 25-100 mg for trazodone) but require higher doses (15-45 mirtazapine and 200+ for trazodone), at which they may instead be activating, to be effective for depression.
Also, I'm not sure what you consider 'incompatible,' and personally I always strive for monotherapy when possible, but in many cases it really isn't possible or appropriate. For example, individuals with bipolar disorder who are currently depressed generally shouldn't be treated with an antidepressant alone because it can precipitate mania. In some cases they can be managed by monotherapy with a mood stabilizer that has relatively better efficacy for depression, such as lamotrigine; but many bipolar patients won't respond to this, and will require both the antidepressant to treat the depressive episode and a mood stabilizer to prevent the antidepressant from flipping them into mania.