Can I connect DDR3 memory device on one DMC and DDR2 memory device on another DMC on ADSP-SC58x/ADSP-2158x?

No, since the DMC0 and DMC1 share the same DCLK and VDD_DMC supply, both the DMC controllers should have the memory from the same DDR mode (DDR3/DDR2/LPDDR).