Those aren't technically double sync inputs, or double inputs for anything else. In each case one is for input, the other is passthrough output; generally top and bottom, respectively. Sometimes with PVMs/BVMs if you look at the schematics you could actually connect two sources, one to the input and one to the output, but you can only ever have one of them powered up at once. But this skips the video termination required.
Your BVM requires 75R termination on each signal line output (if you're using that input) or the image will be too bright and have ghosting blurring it. Get a pack of ten cheap 75ohm BNC terminators from eBay, that way you'll definitely get four that fit easily. Or daisy-chain to a PVM that auto-terminates or something