What you're describing is known as a 'low angle bridle.' The problem with this configuration, where a flexible cable runs nearly horizontally and a load is applied in the middle, is that the tension in the top cable gets very big. In fact, to have the top cable be perfectly horizontal would require infinite force at each end, since its own weight is always going to make it sag a little. Just getting pretty close to horizontal requires a really large force. Because it's generally a bad idea, I don't think you'll find a piece of hardware that looks exactly like what you're imagining.
However, if your trees are tall enough, you can make a more typical bridle (Think a Y shape) where the top wire rope goes up as well as out. In conventional rigging, we try to keep the angle between the top two legs below 90 degrees, although there are some exceptions. If you go that route, you'll want the top piece to actually be two separate pieces of wire rope. Connecting the two bridle legs to the vertical drop you'll want to use either an anchor shackle if it's temporary, or a pear shaped link if you can swage the cable in place. (Obviously since your load moves, make sure to mouse the anchor shackle if you go that route!)