Preservationist replaces poison use and resistances to poison, internal alchemist grants a bonus equal to the resistance to poison to disease. If you combine the archetypes does the internal alchemist get no benefit? Or does he get the usual bonus to disease?
Not all Archetypes are compatible. Usually this is because they modify the same power. This is a rare case of two archetypes which are legal to be put together but one of them effectively removes a benefit of the other.