Should I send in both subject tests to Ivy? I got an 800 math and 670 in physics?
Yes, send them both. If you only send one, they’ll assume (correctly) that you’re hiding a score.
If your a senior who has already applied, then yes. As @sherpa correctly points out, it is exceedingly rare that a person just takes one Subject Test, so sending only one would make the AO think that you are hiding a score. Which you are.
I have already submitted just one score. Should I send other score now?
If you’re specifically referring to Harvard, which is where you placed this thread, then yes.
My older son graduated from Harvard in 2016. He had submitted four subject tests results and only one of the scores was 800. If rest of your application is strong then one low score will not make much of a difference. All the best.