Line 23400 is not shown on the detailed tax summary. You would have to look on the PDF archive of the return to see that line number, as well as the Total split income from Line 68360 of T1206 (if applicable).
Your spouse/partner's net income (Line 23600) is calculated by subtracting social benefit repayments (Line 23500) from their net income before adjustments (Line 23400).
Line 23400 less Line 23500 equals Line 23600. So, if your spouse didn't have any social benefit repayments, then Line 23400 is the same as Line 23600.