When I tried to get a custom filter/combiner, I was told politely that I couldn't afford one. They mostly work if you only want filter one or two channels from the 2nd antenna.
If you want to play around and have a bit of technical know how, browse over to
www.hdtvprimer.com and look for combining antennas, tuning the aiming of them to take advantage of nulls and such.
It's still hit or miss though. But very interesting reading.