The bidding war for National Instruments, or NI (NASDAQ: NATI), is heating up, according to Reuters. Not only is Emerson Electric (NYSE: EMR) still firmly in the frame, but NI's rival Keysight (NYSE: KEYS) and industry peer Fortive (NYSE: FTV) are also believed to be part of the process. What's going on, and what does it mean for the investment proposition for these stocks?