BNC splitter which separates ground and signal?
I haven't seen that particular style of adapter but if you don't have any luck tracking one down a solution I use personally is a pair of BNC receptacle to banana plug adapters along with a BNC to banana socket adapter. They are fairly widely available in a variety of styles such as the following:



Those particular parts are a Pomona 4435175 and Pomona 1894 - I've  given links to them on Element 14 but similar adapters are available from a wide range of manufacturers and suppliers. Just try a search on "BNC banana" to find them, I just did a search on Digikey and it looks like they have a good range as well.