It is very weird to be mixing two different antidepressants. Depending on what your other antidepressant is, it could be dangerous. Were your two medications prescribed by the same doctor, and if not, have the two doctors talked to each other? A very simplistic view is that some antidepressants cause the body to make more serotonin, and others prevent the body from destroying serotonin as quickly. Doing one is fine, but doing both at the same time can lead to serotonin syndrome.
Amitriptyline does have higher risk from overdose than SSRIs, but has much gentler withdrawal symptoms.