As I understand it power consumption from either consumer unit is measured by one meter at present which is provided and is the property of the local supply authority. You dont say what these two consumer units are feeding but if we assume they are feeding two seperate buildings or ground floor and first floor of the same building you have 2 options.
1st option is to apply to the supply authority to have another meter fitted, in which case you would have to set up another consumer account etc in which case each unit would receive its own bill - less complicated. (Supply authority may require certification from yourself or a competent person - check when applying)
2nd option is to purchase and install your own meter either credit or prepayment connected in line with the incoming supply to one of the units.
Consumption recorded on existing meter is total for both units. Deduct consumption from sub-meter to determine individual consumer unit consumptions. More complicated and inclined to lead to disputes especially if each consumer unit has different customer/tenant/user.
You need to talk to the supply authority who may be different from billing agent in this area they will supply a meter and isolator which you can connect a consumer unit to but in other areas they require you to provide the isolator so you need to ask them what they require.
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