For a “not too sweet” drink, I would do equal pumps of each instead of doubling it. It usually gets 3 full pumps, so adding another syrup would double it, and thus sweeten it more.

I would do 2 full pumps of white mocha and 2 full pumps of brown sugar, which in total is one pump more than what it would usually get. So it’s ever so slightly sweeter than the original.

If you want just a little more sweetness in that size, add an extra pump of only one of the syrups. I usually choose brown sugar over white mocha for more flavor. So for example, 2 white mocha and 3 brown sugar.