With the quality of carbon blocks today there really is no advantage to having more than one as long as the one you have is a sub 1 micron like a 0.6 or 0.5 micron Chlorine Guzzler or Matrix type. These are designed to adsorb the chlorine and volatile chemicals out of about 20,000 gallons of tap water at 1 ppm of chlorine residual. Looking at some of the other varities or micron sizes you will see they do not have near that capacity. Regular granular activated carbon cartridges can actually be exhausted after 300 gallons so they always used to provide two of them on the old drinking water systems most of us started out with. Keep in mind when you say 20,000 gallons or 300 gallons that is total water through the RO system including waste so its more like 4,000 good gallons and 16,000 gallons waste or 60 gallons of good and 240 gallons of waste in those two instances.
A 10 micron carbon block may only go 1,000 to 1,5000 gallons, a 5 micron may do 5,000-10,000 gallons etc. I have yet to exhaust a 0.5 micron Chlorine Guzzler type and have been running it for 9 months now without changing it while monitoring for chlorine breakthru with a low level chlorine test kit. I don't know for sure how much water has passed through it since I also use my unit for drinking water, ice maker and other uses besides DI but I plan to start the test over very soon and install a 5/8" water meter in the tap water line and monitor every drop that goes through the system. My one caution is use a low micron rated prefilter in front of it prefferably equal to the carbon so it does not have to act as a second prefilter in addition to a carbon filter which can cause it to plug prematurely. I am presently using a 0.2 micron absolute rated prefilter in front of the 0.5 micron carbon block so it only has to do the job it was intended for.
Any of the units I recommended earlier in this thread should serve you well. I would not try to go any less than them or you get into things like non refillable low capacity DI filters, no pressure gauges or TDS meters, non standard filters etc.
masonicman, you might consider spending another $20 to $30 and get the unit with a full sized refillable DI. It will pay for the difference in your second DI replacement. Those small capacity horizontal DI throw aways don't hold near as much resin so do not filter as well nor as long and cost $18 a pop. Verticals hold 20 oz of resin and can be refilled for as low as $6-$8 a pop with over twice the resin life and better final effluent quality. Just a suggestion.