I am active duty military, I will also be divorced in March 2021 and have not yet cashed my second stimulus check due to separation. I don’t think it is possible for me to cash it since both our

Both spouses would be required to sign the check in order to cash it or deposit it. You should check the information on the check to see when it expires.  It's usually written on the face of the check such as 'valid for 90 days'.

If the check expires then you would have to get a re-issue which could take several weeks.