Feeding two antennas directly into a splitter/combiner doesn't make any sense at all. Its not that the directionality would suffer, since the two directional patterns are pointing in such different directions, so that would be OK. The problem is that in receiving signals off the air the struggle is all about getting the best carrier to noise ratio and putting a combiner/splitter in the wrong way ruins all the hard work that the receiver and antenna designers have done to give you the best carrier to noise ratio (and hence the best sensitivity) possible.
The problem begins with the concept of noise. Noise is caused by many things, not the least of which is brownian motion of atomic particles. We learn that this random movement of atomic particles generates noise power within conductors and semi-conductors and resistors. This noise is something we cannot escape, it is in everything. It is in the coax cable coming from the antenna. When you receive a signal, your ability to decode its meaning, the intelligability if you will, depends on how much noise is corrupting that signal. Just like when you are trying to hear someone speaking, if the noise around you is too strong you cannot understand what they are saying even though the sound they make is entering your ears just fine.
The amount of noise from the antenna is predictable and we call the amount of noise power the "noise floor". It is called a floor because it is big and relatively flat and forms the bottom of our intelligability range.
So the noise power from your antenna is at a fixed and irreducable level. When you receive a signal, you need to receive more signal power than this noise power. But when you put a passive splitter/combiner in your coax, you decrease this signal power but you do not decrease the noise power. Why? Because the splitter output has the same amount of noise power or noise floor as the antenna output. Yet, it will attenuate the signal you are trying to hear because combiners are not perfect, they suffer some losses. So what you are doing is decreasing the ratio of signal to noise (the well named "signal to noise ratio" or SNR) when you put a combiner/splitter on your antenna feedline. You can never get the signal back with all the amplifiers in the world because amplifiers cannot remove that noise power, they can only add noise power.
The effect of putting a combiner in the antenna feedline is to lower your sensitivity by about 3.3 dB. That's a big hit. So, don't do it.
If you must combine in this way, the way to do it is to first amplify the signal from each antenna individually, using an amplifier with a very low noise figure. Now you may combine the two amplified outputs together in a combiner and you won't suffer that big loss of sensitivity.
It is likely that you can buy amplified splitters but these work in the reverse way to what you want. You need an "amplified combiner" with each of the two inputs first amplified then combined. Not sure you can buy this sort of thing. So buy them as separate elements and hook them up yourself. Minor problem of how to get power to the amps, but you can feed DC up the coax if you use a splitter that can pass DC.