In general, it appears that psoriasis is more related to the immune system issues (as compared to seborrheic dermatitis). As a result, many of the documented treatment options are typically more systemic.
One of the most promising topical treatment approaches for psoriasis appears to be fumaric acid esters, but one would need to closely examine the safety information before such treatment.
If you suffer from both seborrheic dermatitis and psoriasis, perhaps your immune system may be struggling. Improper immune system function and these two conditions can often go hand in hand. Strengthening and restoring immune system function could lead to substantial improvements of both conditions.