In answer to your first question, oftentimes psychiatrists begin with a simple regimen of a using 1 medication to treat most symptoms in a mental condition. Later, some psychiatrists either increase the dosage of the medications and if this does not work, a change of medication is done OR add other medications to potentiate the effects of the other medications. Psych meds cannot be stopped abruptly. They often need to overlap to decrease the incidence of withdrawal effects from the medications. (I wonder if that is the strategy your husband's psychiatrist is planning to do).
You ask if it is common practice to have multiple medications, psychiatrists strategy differ on the type of patients they see. Some psychiatrists have cases that respond to monotherapy and other psychiatrists have patients that do not respond positively to different medication regimen.
Like medications, the theories in psychology differ in the beginnings of depression. Some psychologists believe it stems from childhood. Some believe it is a product of their present adult behavior. Either theories are neither right or wrong. Both have different type of therapeutic intervention.