Generally dual video cards are envisioned by most consumers as 2 identical cards for NVIDIA SLI or AMD CFX. Windows 10 and DX12 however have changed the way video cards can be used.
For older games a high end card can handle games through DX11 easily at 1920×1080. Dual cards are only needed with 3820×2160 panels with some extremely demanding games.
Both of these cards are NVIDIA Kepler based. The GT 640 works fine in an x4 slot for general purpose use. There is enough bandwidth with PCI Express 2.0 that video cards are not appreciably bottlenecked.
The GTX 660 Ti has 7 SMX units and the GT 640 has 2 SMX units. DX12 uses tiles instead of alternate frame rendering. DX12 motivates improved GPU performance as games become more demanding.
NVIDIA supports using a second video cards for PhysX processing. There are over 100 titles that use PhysX so this configuration will provide some performance gains but generally the difference will be negligible.
By default the NVIDIA driver defaults to the GT 640 but the settings are by default on automatic. This way if the card is needed for tile rendering it can be allocated.
NVIDIA is supporting WDDM 2.1 and DX12 for owners of Kepler and above. This includes Maxwell, Pascal and Volta cards.