I am running two canister filters on one tank right now. I wanted more flow and had an extra sitting around. I simply just put a T in on each side, one between the inflow of both canisters and one after the outflow. Many will say it makes more sense to run two filters separately, and I would agree, but my tank is on the smaller side so the extra inflow/outflow would be pretty evident, plus I didn't have a lot of space to plumb this outside of the tank so I just decided to go for it.
So far I haven't experienced anything that leads me to believe this is a bad idea. Many say running them in parallel, meaning the outflow of one goes straight into the other can damage the pump which I don't know to be true or not. Many others say it just doesn't up the flow much, basically the first canister is still a restriction on the first, or the second, now being pressurized, can't keep up. I don't know if this is true either, I imagine it all has some merit but I don't know in practice.
The only downside I have identified is that with only one outflow, you cannot easily improve dead spots. Sure, you get more flow but it still is from one place. Because I am using lily pipes, rather than a spray bar, I imagine this is more of a problem with the type of outflow I have. The inflow can also clog more quickly/easily when I have melting or die off/high organic matter but not enough to stop or slow flow enough to worry.
My long term concern is how easy this will be to clean. I don't clean my canisters often so I have yet to try. Two major issues that may arise. One is, between the tubing and stand size, there is not much room to work. It may be simply challenging to pull them out routinely. Second is priming them. One of my canisters primes easily, the other does not. The first time I set it up, I primed the one that has a priming function which filled the other decently, then I found that when I turned that one off, the opposite filter did remove the existing air, well most of it. I don't believe Sun Sun's are easy to prime, the only reason I bring that up. Having one canister that has a priming function makes this work OK, I don't know I would get this working without that, if I lost gravity feed.