I'm planning on using 2 receivers that operate on the same frequency. So there's no confusion I'm talking about WiFi signals (but I'm trying to do some cool amateur radio stuff with it!) so the frequency is 2.4GHz. One of the main things I'm working on is building a nice big antenna (high gain) to be able to have a usable signal over more than 10s of meters. Now ideally, since these two receivers will have the same frequency, I could place both receivers at the focal point of the antenna and have the one dish with 2 receivers - but I'm worried that there will be a loss of signal since the antennas are blocking each other to some extent. Is this the case? I'm looking for an experienced person to give me there best guess really!
As your comment mentions, since you're transmitting also, you will have issues.
Placing two antennas within ~1/2 wavelength of each other will cause them to inductively couple - effectively connecting themselves to each other. This results in detuning both antennas, as well as the high power Tx RF getting routed back down the other antenna, possibly damaging the receiver. I did this once with 2m mobile rigs - I had my HT connected to a magmount placed too close to the antenna for my 50w mobile, resulting in a fried protection diode in the HT.
As others have asked, what is your goal in having two receivers? If you're trying to run two different networks, why not just overlay a virtual SSID on the same AP?
73 de KD8EVL
Main factor for transmission distance is so-called effective heights of antennas on both sides. Basically it is about how high antennas are located - the higher is the better.
Another important factor is lack of obstacles in line of sight - hills, trees, buildings, etc.
Length/size of antenna elements is important only to extent you keep original antenna design. No special need to make it bigger.
At some height of antenna you will need protection from wind (well-engineered mechanical mount) and lightings (grounding within electric code rules) - both are safety protections and law-regulated across most of world.
If your antennas are directed, it is important you orient them on each other along direct visibility.
Your idea about two receivers is waste of time/effort/money - you can do even hundreds of meters without it, on basic equipment, if you keep rules above done. Area of radiocommunications (antennas, receivers, transmitters, etc) is well-studied during XX century due to high impact on military operations - it is unlikely that coolness of your idea makes something new.
You can rather spend some time experimenting in making your own antenna from Pringle chips can, so called Cantenna - it will be cheaper and have more impact. Reading books on topic in local library will give you even more boost.
You can vertically separate the two receiver vertically, one directly on top of the other. The transmitter are very ineffective at transmitting directly above or below. Almost all the power is horizontal to the antenna. The next thing is to also use the 5Ghz band 802.11N/AC of WIFI if possible on one of the routers to further reduce interference. If you have to use 2.4 802.11B/G for both you can manually pick two channels that are the farthest apart in frequency.