Assuming Max 2015 the SPs are cumulative - SP2 includes all the fixes from SP1. There is no harm in installing both (in order, of course) as that is what most users would have done.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).