You should not combine two antennas on the same band naively. The signals won't just add. What you get will depend on the amplitude, phase and noise of each of the antennas when you mix them together. No guarantee that the signals will add constructively, and your noise level could be higher than with either antenna by itself.
If you want to have two antennas pointed in different directions, the simplest solution is an A/B switch for the antennas. Just switch to the antenna you want to watch the channel that is best from that antenna.
http://www.solidsignal.com/pview.asp?p=pvab95
The RCA preamp is not meant to combine two UHF antennas. It is meant to combine a VHF antenna with a UHF antenna. If you plug the Eagle-Aspen into the UHF port, you will only get UHF from the Eagle-Aspen. If you plug the Winegard into the VHF port, you will only get VHF from the Winegard.