jl35, I think both audiokinesis and erik are right from the perspective of diminishing nodes throughout the room. But, there are other considerations particularly if what you care about most is the listening position.
I would like to add several other points. The harder you push a subwoofer the more distortion you get. The placement that is most efficient is in corners, next is up against a wall. Timing is very important with subwoofers. It is important that the wave front from the subwoofers gets to you at the same time as the wave front from the main speakers. This is easily demonstrated with kettle and bass drums. You feel this more than hear it. In a system that dose not have digital bass management the most likely way for you to achieve this (but no guarantee) is to place the subwoofer equidistant with the main speakers from the listening position. The only practical way to do this with two subwoofers is to put them between the main speakers with all speakers on the same radius from the listening position with the subs right up against the wall for efficiency.
This will obviously not produce the best nodal behavior throughout the room but it can provide the best performance at the listening position. Instead of "crawling" to position the subwoofer you "crawl" to position the listening position! Walk forwards and backwards from your listening position and find the spot with the best bass balance to your ear. You will be surprised how much the frequency response will change with just a few feet of movement. Use a song with a lot of synthesized low bass or undamped bass drum. The first cut on Brittany Howard's new record "Jaime" has a great undamped bass drum, perfect for this use. You can feel the air move in the room.
It is also a good idea once you have everything set up to reverse polarity on both subwoofers to make sure nothing is inverted. The polarity with the most bass is the right one.
It is always a good idea to use a high pass filter on the main speakers. If you do not already have a high pass filter you can make a 1st order one just by placing a capacitor in series with the positive leg of the amplifier's input. The value is chosen based on the crossover frequency and the input impedance of the amplifier. You can easily find the math online.
