Merger of two GSTIN in the same State - Procedure to be followed - Goods and Services Tax - GST

There are two registrations in one State. There is one manufacturing unit under each of the registrations. To merger both the GSTINs, is it required that - (i) finished goods to be sold by raising tax invoice from one unit to another unit; (ii) stores and spares to be sold by raising tax invoice from one unit to another unit and (iii) plant and machinery to be sold by raising tax invoice from unit to another unit. The second unit (which is going to be continued) can avail ITC. But, transfer of stores and spares, plant and machinery is a cumbersome task, involving huge documentation. Since both the units are of same company, is it required to transfer stores spares and Plant machinery also by raising tax invoice. Experts are requested to give their views.