I don’t have my UHF and VHF antennas stacked on top of each other because I found out that my UHF signals come in about 2 feet to the left of my VHF signals. I didn’t even mount my antennas high up in the attic. They are both near the floor mounted on wooden poles\stands (like my FM coat hanger) three feet apart from each other.
Another tip. You can add to the VHF dipole’s reception by sticking a copper pipe or tube (or even 4 or 6 gauge copper wire) as a reflector about 70 inches long, 20 inches directly behind the antenna (on another pole). That can add up to 30% more power to the antenna as it bounces the signal back onto the antenna.
Or a 4" by 70" long strand of chicken wire will act as a good reflector. Or a long roll of aluminum kitchen foil taped to cardboard.
Yes, that RCA preamp with separate VHF and UHF inputs will combine the two antennas. Make sure the switch on the bottom reads “Separate VHF UHF inputs” and not combined.