There's a good primer on capture cards
here.
My recommendation would be either a couple of
C127's, or a couple of
C985's, depending on your budget and needs. The extra money for the C985 gives you two relevant features:
A built-in hardware scaler, which allows the device to give a different resolution to OBS than what's coming out of the source (PC). The major thing this allows you to do is achieve 60FPS from 1080p sources by scaling them down to 720p. Neither of these devices will allow you to capture full-resolution 60FPS 1080p, they both capture 1080p sources at 30FPS (regardless of if the source is running 30FPS or 60FPS).
Pass-through: This allows you to plug a monitor into the second HDMI port of the C985, such that you can view the signal that's being sent to the card. To do this with a C127, you'd need to pipe it through
a splitter (still lower combined cost, just slighting increased wiring mess). If your displays and source computers are near each other and further from the capture computer, this means running two long HDMI cables there and back for each source computer, versus two short cables at each computer + splitter combo, and one long cable to the capture computer; something to keep in mind, depending on the layout of the machines. All that said, if your source computers both have a second output on their graphics card, you can just clone the displays and not need a splitter with either card.
I personally use two C127's for this purpose. They work well for me, but I exclusively stream at 30FPS so the 1080p60 capture limitation doesn't come into play, plus my layouts require the sources to be further from the capture rig, so the splitter configuration is preferable.
