I assume you are talking about two identical yagis pointed in the same direction.
How they are combined depends upon the feedpoint impedance.
Usually, if the 2 antennas have a 300 ohm impedance feedpoint, you would connect each of the 4 to 1 baluns to a 75 ohm splitter/combiner using equal lengths of coax. It is important that the two antennas be properly phased together. If you get less gain with two antennas than with one, reverse the 300 ohm connections at the antenna of ONE of the baluns.
Phasing together two yagis with 300 ohm twinlead is not a good idea because its impedance changes a lot when it gets wet or covered with ice and it flaps around in the wind which also changes its impedance as it moves closer and further away from metal objects.
I have no personal experience with the Winegard AP 4800, so I can't comment on it except to say that Winegard has a good reputation. I like my CM7777, but it has a tendency to overload with very strong signals. The Winegard HDP-269 has less gain, but resists overload better. The length of your coax feedline is also a factor to consider when you select a preamp. If you look at the tvfool results posted by jtbell in post #3 you will see that the CFPL CH 10 signal pwr is -4.7 dBm and marked in red which indicates possible overload.
Before we give any more answers about your desire to receive US stations, we really need to see what your tvfool.com results are based on your exact coordinates. This is because London ON has some very strong transmitters near the center of the city. If you live North of the center of London your antenna will be pointed at these very strong local stations that will make it difficult to receive US stations. It would be a lot easier if you lived well South of the center of London because the back of your antenna would then face the strong local stations.