I have a Master's in a non - STEM degree, and almost finishing my second master's in a STEM degree. I have already used my master's F-1 OPT ( 12 months). I am wondering if I can still apply for the the F-1 OPT STEM extension using my second Master's in a STEM degree.
If I understand you correctly, you first did a (non-STEM) master's degree #1, followed by 12 months of OPT, and later started your second (STEM) master's degree #2. Then the answer is no. You need to be on OPT while applying for the STEM OPT extension. Yet you can't be on OPT, since you used up the 12 months from this degree level already.
The limiting factor is that OPT can be authorized for only 12 months total per degree level. If you had known you were going to get a second master's in a STEM field, it would have been wise to get your first OPT for less than 12 months. For example, if your first OPT had been authorized for 9 months at master's level, you could apply for 3 months of OPT now, and then the 24 month STEM extension. Unfortunately this option isn't available once you've already had 12 months of OPT at your current degree level.