So I've recently noticed (I haven't really kept tabs on this before so this is the first time I've noticed this) that in some applications, only one of my GPU's (the first one) is actually exerting itself. The second one still slightly moves in clock speed and voltage but only the first card is actually clocking appropriately for said applications. Is this normal? I'm not concerned because they both kick in just fine for other applications, I'm just curious if SLI doesn't actually start to use both cards until it needs to. I have two GTX 770's, one of which outputs to my monitor and the other is auto-selected by the nvidia control panel as the Phys-x processor.
I think the game has to have a SLI profile to really use SLI so if a game does not it does not really use SLI. Or at least that is the way I have read it before. I know on some of my games that both cards are being used but only about %50 on both cards while on other games almost %100 but even on the ones that are only about half on both cards my frame rate are still great.
First - you need to enable SLi in the NVidia control panel
Second - the GeForce drivers have preset settings for a large number of available games which defines how the SLi works, you can add games to this as well edit existing settings. Some profiles disable SLi (resort to single card setup) because the game doesn't support it. If a profile doesn't exist for your game it will disable SLi but you can create a profile for it and tinker with the different SLi setups until you get one you're happy with.
My best suggestion would be to update your drivers and re-enable SLi in the NVidia control panel.