How to cause Nvidia GeForce 6150 and Nvidia GeForce 8500 work together?

How to cause Nvidia GeForce 6150 and Nvidia GeForce 8500 work together?

You don't, they run a different clock speeds...you can not do SLI beside those two graphics cards.
Mike is right, Also if you wanted an SLI configuration, you would necessitate two of the SAME videocard. Sorry to disappoint.
That is not doable, as Nvidia's SLI Technology requires two identical cards. You also will enjoy have a motherboard that have two PCI-Express 16x slots, designed for SLI.