With it being a newer model, safest bet is to not void your warranty with an unsupported configuration.  If anything else went wrong, you'd have to put the configuration back to pursue a warranty claim.  Also with the 3660 being newer, sticking with supported configurations is also a safer bet  Overall, Dell's have been hit and miss with mixing RAM.  Some don't even tolerate mixing brands of RAM.  If you tried the configuration in question, all other specs would have to be the same - brand, speed, and volts.  Also, non-ECC and ECC especially can't be mixed.
I have Dell Precision 3660 with 16G(=8G*2) memory module. I want to add 32G memory (=16G*2).  Is it okay to run 48G (=8G*2+16G*2)? Dell manual does not show this configuration.
Precision 3660 Mixing Memory Module