There's really no relation between number of engines and APF.
The effectiveness of APF (and whether to increase/decrease) is going to be determined by your queries, fragmentation of your data, cache configurations and disk subsystem abilities.
I'd be looking at sp_sysmon to assist with determining the effectiveness of APF.
---------------------
Doubling the number of engines?
Hmmmm ... are there plans to double the workload, or is this an attempt at improving the performance of the current workload? If the latter ... I hope someone's already done some aggressive P&T work (query tuning, process redesign, server/cache configurations, etc) and determined that increasing cpu counts will be more effective than adding memory and/or SSDs ...