I think that the best way to determine this is to actually observe the users performing their tasks. If they have to switch frequently between apps to perform a certain task, your best bet would be to combine the apps into a single one in order to facilitate the workflow.
One of the adverse effects of the multiple apps I can think of is that the apps may get 'reset' by the OS occasionally. You may consider investigating this aspect further, as it may also have unexpected effects on the workflow.
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