If you search Solid Signal for "combiner"
you'll find pages and pages of what seem like the very same thing. Not only that, you'll see things that look very much like splitters, and some that are labeled splitter/combiners. It can be very confusing to try to find what you're looking for. Here's a quick tutorial:
The basic splitter/combiner
This is as simple as a combiner can get. This Channel Plus