Welcome to the forum. Your dual drop table sounds like a fun build. I custom built the 1st (that I know of) Dynatorch Dual Drop table design in 2009. I already had a Dynatorch table I purchased in 2005. I redesigned the existing gantry and table into a Dual Drop and added several new features. I might add that Leon Drake, who is the engineer at Dynatorch gave me some valuable tips during my redesign process. Here is a link to my build;
http://www.cnczone.com/forums/dynato...h_upgrade.html (NEW BUILD / Dynatorch Upgrade)
Dynatorch recently introduced their own Dual Drop table they call the XLR8 Dual Drop. Most likely they have provided you with a set of table plans for the Dual Drop Gantry. I would suggest that you don't deviate from the plans. Leon has been designing and building plasma tables for many years so I'm sure the plans incorporate tested in the real world proven designs.
To address some of your questions;
1. Using Dual Linear rails on each side?
Dual linear rails on each side of the X or long axis is something you normally see on a extreme high end industrial (translated extreme high cost) plasma machines. These machines have machine tool type tolerances like Haas Verticle Machining Center would.
For the home or small shop builder installing 2 oposing linear rails spaced 6' apart in the verticle and horozontal planes within a couple of thousands of an inch of each other is a challenge to say the least. Can it be done? Some say yes. I say its not worth the effort.
An industrial builder is able to acheive this by 1st placing their welded table in an oven to stress relieve the welded frame and then by putting the frame in a bridge milling machine (12' or more travels) and machining 2 flat linear rail mounting surfaces (way registers) on the welded frame. This ensures each linear rail is in perfect alignment with the other. This process also adds big money to the cost of the build.
2 other methods can be used to address the alignment problem.
1. Use one fixed linear rail on the master side and a V rail with a 'floating wheel' on the slave side that can move back and forth to compensate for any misalignment. This is what Dynatorch uses. This method works great and I use it on my machine as well. The slave motor holds the gantry in perfect alignment while the floating V wheel compensates for misalignment between the master rail and the slave side rail.
2. Adjustable floating rail mounts. I attached a picture of a Koike plasma table with adjustable rail mounts so you can get an idea.
My suggestion is to stick with the floating V rail design which has an added benefit of being self cleaning.
Aluminum water tray?
I would stay away of the aluminum tray for a couple of reasons. Aluminum corrodes much quicker that steel. You will need some type of additive for the water which after cutting awhile can become an interesting mix of a brew.
You also have to consider the material slat support system which will be submerged in your water tank will be steel. It will have to be able to hold a great deal of weight and most likely be welded or attached to the tank its self. Perhaps WSS who frequents the forum will chime in to give you some tips on the holding tank system as he built a killer set up for his Dynatorch table.
Hydraulic lift cylinders for the slats?
If you are concerned about gently loading your material on the table I would consider using some magnets with chains and suspending the material from the forks when loading. Also be aware cutting with water directly under the sheet is an invitation for a unwanted shower for you and your expensive equipment. Much like pointing a compressed air hose at a pool of water 2" away! I like to run my water level about 4" below the sheet. At that height it provides a happy medium between smoke and dust removal and splashing water. In addition splashing water on your material certainly is not a plus for cut quality.
Another tip; check out the We-cim package Dynatorch offers as it will make your life much easier when you start creating those parts on your new table.
Have fun! I will watching your build.