Don't run Shadowplay and just use OBS's NVENC. And run two instances of OBS at the same time with a -multi switch on your Windows shortcut targets. NVENC is nearly free as far as CPU and GPU costs go, and is great for saving to disk at a high bitrate, so it'll hardly affect your normal software OBS to Twitch performance.
By the way, the new Maxwell chip on the GTX 750 and 750Ti is brilliant in that it can save to disk at 1080p60 at the highest quality NVENC setting, which the older Kepler NVENC couldn't do (at least my Kepler couldn't).