Dual channel is applicable to the amount of ram on each channel.
In theory, if you add 4gb, 8gb of your 20gb will operate in dual channel mode.
But... ryzen is picky about ram.
If you want dual channel for 16gb, buy a 2 x 8gb kit of supported ram.
Ram must be matched.
Simply adding another 16gb stick is not 100% guaranteed to work.
It might, but what is your plan B if it does not??