Trying to combine same-band antennas using a $5 splitter is impossible to predict, it's purely potluck. Cable companies use specialized combiners that can cost hundreds
PER CHANNEL to do this task. The only inexpensive solution that works 100% of the time is an A-B switch. A-B switches, of course, are inconvenient for DVRs as well as a number of TV set tuners that force complete re-scans.
GroundUrMast posted an article a while back where he detailed using separate notwork-based tuners with one dedicated to each antenna. Perhaps he'll highlight it.
Wish there was a simple solution, but there isn't. You may want to see this tutorial:
http://www.hdtvprimer.com/ANTENNAS/merging.html