You can use this to combine two antennas as well, or is it only a splitter?

Yes, I used it to combine 2 indoor antennas. I had an antenna (where uhf came in good) on one wall and another antenna (where vhf came in good) on another wall. Using this splitter made it possible to get all the channels I wanted. I went from getting 15 channels with one antenna to getting 32 using the splitter to combine the antennas.

No, this is only a splitter.