I live just east of Toronto, which means I have Toronto in one direction and Buffalo in another. If I aim my antenna at either one I get good signals from that direction, but lose the other.
How far away are you from each market? I sit in between 2 cities and use a leaf style antenna to get both.
I have a Mohu Sky. I have most channels in one direction but two in the opposite direction. Using a omni-directional antenna could resolve your issue as it did mine. As @jbanks25 asked what is the distance? That will determine the strength of antenna you need. All antennas are different and there are a lot of factors that can get in the way. Such as buildings, trees, etc. So, just because you are 40 miles and have an antenna good for 60 miles, those factors can affect the strength of the signal.
Yep it is being done.
http://www.tvantenna.com/support/tutorials/combining.html
http://forums.solidsignal.com/showthread.php/4163-TIP-Combine-two-antennas-for-even-better-reception
Just use a combiner 2 in 1 out
In my garage attic, I replaced an omnidirectional RV antenna (in the floor) with a Mohu Leaf (30) connected immediately to a drop amp (+15dB). This increased my weakest station (30%) to 70%. The leaf is small enough that finding the right place and securing it is a lot easier than that bulky RV disc.
I think they should offer a 2+2 Tuner Tablo TV, which would solve multiple antenna issues, and co-channel interference.
I guess a 2+2 tuner would work as long as the software specifically allows you to set which channels come from which tuner.
@PiX64 is correct and it is a very cost effective solution. I have four antennas in my attic (long story) and they are all combined into a single signal running to my garage via a single cable. With analog TV signals you had to be very careful when you did this but not so with digital signals - or so I’m told. Regardless, I find it works great.
@Galroc - There are a lot of ways we could do this but hardware variants are less of a focus than providing more features for the ones we already have 
Re: The same tuner is in the 2 and the 4, we just use 2 vs. 4 of the tuners in the 2-Tuner version.
I predict "www.hackingtablo.tv" to be live by the end of the week.
I predict “www.hackingtablo.tv” to be live by the end of the week.
3 seconds between swallowing coffee and seeing that.
Somehow I missed all of these replies.
The problem with using a combiner is that you lose 50% of the signal. So if you combine two antennas pointing in the same direction you get 1.5x the signal, not 2x. If you point them in two different directions, you get ~65% of both. Now for the Toronto-side this is not an issue, I could probably use a leaf antenna and get everything I need. But for Buffalo I barely get the signals I want with a single 4-bay. An 8-bay doesn’t help because the signals are two spread out and an 8-bay reduces the horizontal beamwidth.
Two separate inputs really is the way to go. One solution would be to use two Tablos, but it appears the client software cannot handle this case, which is somewhat annoying (you can only connect to one at a time). So a split-tuner solution is the only way to go.
Has anyone actually attempted this?
You might want to study how this person did it. He gets all the channels from both Buffalo and Toronto from Hamilton at 80 to 100% signal strength with attic antennas going into one entertainment server. Multiple antennas are doable for great reception. He provides a system diagram for all his antennas, devices, interfaces and connections for building a multi-site environment.
So can you guys offer a purchased software update for people who want 4 tuners instead of 2 later on?