glofish,
The manual (Part A) states actions in the event of an engine failure and continued flight, most crew know this usually by memory however it would obviously be reviewed in a real event.
Once all those boxes are ticked (consider a second engine failure, have other systems been affected other than what's a result of the engine shutdown, fuel req on 3, fuel req on 2, terrain on 3, terrain on 2, alternates/weather etc including the commander considers it safe to do so).
I'm sure most concerns would have been raised and discussed simply by going through Part A, both crew would clearly be in the same loop and if the 'go' boxes can't be met the prowess of a 4 holer won't cut it either. If someone thinks it's unsafe to continue on 3 they better not even leave the gate when boarding a twin. The rules that allow a twin to depart the gate are just like the rules that allow a 3-4 engine A/C with an engine failure to continue.