I got my I-751 application and N-400 at the same time, but in my case I did not get a combo interview at CSC (California),
07-Feb-18- I filed my I-751
06-Mar-18 I got received Number and extension for 12 months ( I-751 Form)
16-Nov-18 I got extension for 18 months (I-751 Form)
07-Jan-19 Apply for my N-400 Online
08-Jan-19 I got Received Number (N-400 Form)
12-Jan-19 Scheduled for Biometrics (N-400 Form)
30-Jan-19 Reviewed my Biometrics (N-400 Form)
01-May-19 Scheduled my Interview (N-400 Form)
05-Jun-19 N-400 Interview ( No Combo Interview) - The IO told me that he did not received my I-751 Documents from CSC so he couldn't take a decision on both of my cases he continued with the N-400 and I pass it but I had to wait until the IO receive my I-751 Documents that it will take 2 to 3 weeks to get both of my cases approved and that never happened.
28-Jun-19 Received a letter Scheduled for Biometrics for my I-751
09-Jul-19 Biometrics appointment (I-751 Form)
03-Sep-19 I received an RFE from USCIS for my I-751
19-Sep-19 I send my RFE Documents (I-751 Form)
25-Sep-19 I got my appointment to get a Stamp on my passport for 1 year extension (I-751 Form)
10-Oct-19 My I-751 was Approved
15-Oct-19 I received my Green card by USPS (I-751 Form)
21-Oct-19 Im still waiting for my N-400 Decision
If you have or had a similar case please let me know what is your experience had your hand, I will like to know the time processing time on this type of case, thank you and good luck.