Sorry about the delay. Had to check on a flight into someplace I'm not supposed to mention.
The biggest advantage is that you only have to wire them together once. Based on what I got from earlier, you run the cables from your mixer to the amps every time you set up. This way if you rack them together you only have to connect them together one time and leave them and then only have to run one set of cables from your mixer instead of two.
There is also a setting on your 1400 to bridge the amp. Which basically joins the both sides of your amp into one with a single output (usually A or 1 side). Run that output to one of your subs and then out of that sub into the other sub. That will lower the resistance because of being parallelled and increase your output while putting less load on your amplifier. It will work harder but more efficiently if that makes any sense.
You can do the same with the RMA800 but I believe you will have to use the binding posts on that amp to make that work. I have an RMA 1600 that I used to use that way and I think thats how I had to do it. Either way it will do the same as the 1400 power wise at that point. My recommendation would be to leave the outputs from it alone (don't bridge it and keep running stereo out of it the way you have been (one cable to each speaker) so you still have stereo if you want it. Tops are fine to run stereo but its really not that useful for subs (most audiences can't hear it).
I also noticed that your particular amp (1400) has several small knobs and switches on the back for setting output levels and input settings. You may not need a crossover after all. If there is a knob for low pass filter that will do pretty much the same thing and you don't have to buy any more outboard gear.
Have a look at the manual online
http://www.mackie.com/pdf/m1400_m1400i_om.pdf and do some studying. There looks to be a ton of available options for that particular model. The back of your subs should have a frequency range on them. Use that as your baseline for setting the switches. Not 100% sure all of this will work exactly as described but it looks promising. Be sure to read all of the manual before you risk a show on this. Sometimes bridging an amp shuts off one of its inputs and would leave you with only left channel (or maybe just right).
Hope that helps.
By the way, I was just in Bangor late last week on my way out here to sand land. How far is that from you?