Coupling off grid and on grid system together

What are the drawbacks of connecting or coupling an on grid and off grid system together. The panels are shared. When the DC cables come from the panels there are 2 strings that goto the on grid inverter and from the 2 strings one string from the off grid system is coupled with a string from the on grid inverter. I know this is not the right way, but i need a solid reason why it is not a good idea to do this.

You cannot connect one string from one inverter to another inverter, it could damage the inverter! What do you mean by ‘coupled’? Do you mean connected in parallel?

The MPPT (maximum power point tracker) on the inverter(s) cannot function properly if it is connected to another inverter. The current can only flow to one of the inverters, it cannot be shared.

What inverters are you using?