its not recommended to mix two cards with different memory. you may have to adjust it manually, as per what NVidia has on their website:
"When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. However, while it is not recommended, NVIDIA does offer the flexibility to run graphics cards with different sized memory by using CoolBits. Using CoolBits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate them together in SLI mode. When dissimilar memory sizes are enabled to work together using CoolBits, the effective memory size for each card becomes the smaller of the memory sizes."
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