I am in the same boat with MSC. I filed i-751for my wife on May 04 2020. Received NOA and 18 months extension in June 2020 . Since then we did not hear anything from USCIS. No Biometrics !!!
I dont think so,people do the same,she can get both approve at the same or just a little after tye i175 ,i would ve been eligible but my wife died and i have to wait 5 years to file.
Just recwive q letter for biometric appointment on 9th of april. Though i live in florida but i am moving to texas next month after the biometric appointment