TOPS Coregistration is a chain of processes which can only be applied to raw images. It already contains the Split operator so you cannot input images which are already split.
Make sure you reduce the number of bursts in the Split operator to the desired area to reduce the file size. Example here: Apply orbit file error in S1-TOPS Coregistration - #13 by ABraun
After Back Geocoding you have a coregistered stack (same output as TOPS Coregistration).
But please be aware of the limitations regarding DEM computation from Sentinel-1 data: Phase to elevation error - #5 by qglaude