Hi Albert,
I gathered the app to app navigation is not possible at this moment. At best, My Inbox application can show 'Task Description' of different work-items / tasks from the SAP Inbox, but it does not posses that intelligence yet to change the layout to display 'Approve PO' or 'Approve Req' Fiori apps.
In my humble opinion, My Inbox app is awesome for custom developed workflow and the actions can be captured through BADI (custom attributes). I would rather use a dedicated Fiori app for PO or PR approval.
Hope this helps.
Rohan