I’m submitting my ACT score to Brown, and although 2 subject tests are only required if you’re sending SAT I, can I send just one SAT II (Math II- 770) on top of my ACT score?
Thanks 
Last year I sent them my act, and 3 sat II, and didn’t send them my sat I. Unless they changed anything from last year, you should be fine; the act fulfills the requirement and the subject test gives them a little more to consider.
The Math II score can only help; the only concern is that Math II has a particularly harsh curve, so a 770, while well above average, is likely in the 70s of percentile.
All in all, it should still help.
You can if you want, but a 770 on the Math 2 is a rather unimpressive score. It might not be a good idea.
A 770 is a fine Math 2 score, especially if you’re not applying as a STEM major.