Can you add another stove to an existing circuit?
Nope

NEC 210.23(C) limits 40A and 50A multioutlet branch circuits in dwelling units to powering fastened in place (i.e. built-in) cooking appliances, not freestanding ones.  (A regular range circuit only has a single outlet, so it is governed by NEC 210.22 instead.)

Your 40A branch circuit could power either range, though, as a 13kW nameplate range comes out as 8.4kW of demand-factored load when you apply Table 220.55, note 1 to it, and NEC 422.10(A), paragraph 4 expressly permits the table 220.55 branch factors to be applied to household cooking appliance circuits.