I am teaching myself sight singing right now, and I know how to sing single note(the left one as pictured) on both treble clef and bass clef but get stuck in multiple notes(the right one as pictured). Assuming that is the bass clef, do I need to sing out as mi do at the same time or just either one of them? Thanks.
If we exclude the cases where the score requests Overtone singing (very rare, quite specialized), it must mean that there are multiple voices (which implies multiple people) singing at the same time.
Here's some random example from Wikipedia (from the page Four-part harmony):
In it you can see 4 voices that would be sung by at least 4 different people at the same time.
Generally speaking (or singing!), a person only sings one note at a time. It's possible to sing two, but the vast majority would only sing one. I guess you have the choice here, sing mi, then do; or mi then mi again.