We certainly do not recommend connecting two „strings“ together at the high DC level. The LiFePO4 cells have very small internal resistance and connecting cells at high level voltage (e.g. one pack is 53V, the other pack is just 48V) will cause extreme currents that may cause damage to the cells. Also the operation and the performance of the two „string connected“ packs is quite hazardous.

The only option is connecting the cells (of the same capacity) at the single cell level. However for the connection at the cell level the cells should be of the same type, model and capacity. See details here.

Also for the BMS: you still need just one BMS for a pack that will be made of the single cell level interconnected cells.