I have been doing two kits almost since I started home brewing, as I never like the results I got when I used sugar or dextrose with one can of hopped extract. The next day I always felt I had been drinking nasty cask wine (=a headache).
As plain liquid malt extracts are more expensive than hopped extracts (over $11 Aus as opposed to around $9 Aus hopped) and I really enjoy very bitter beers like authentic Pilsners and Bavarian Lagers etc, it just seemed to make good economic sense. I nearly always boil the cans of extract with 2-4 litres of water though, which I guess, reduces the bitterness a little because it changes any aroma hop flavours.
I just do a bit of math to bring the brew up to 23 litres. Usually 2-4litres of hot + the two cans contents (3.4kg which approximately equals 3.4litres) plus whatever cold tap water is required to make up to 23 litres of brew. I find I usually have to try and reduce the temperature of the wort before pitching the yeast and I have only ever pitched one yeast packet, as the other is kept for backup. Admittedly though, I sometimes keep a cup or so of extract back to boil up later with a little water to add to the bottles for priming although dextrose is almost okay for this too. Generally, I like all malt brews so I formulate that cup (or 0.25 litres) into the equation.
If you’re really hard up though, you can get the same result with just one can by taking the 23 litre formula above and halving it all. So you get one can at 1.7kg + 1-2 litres of hot and whatever you need to bring the brew up to around 11.5 litres. If you don't like the strong bitterness you can also add an additional 1-2 litres of water which mellows the flavour a little. In the 23 litre version you could add a little more water, but fermenter space is sometime limited. Even if you’re not hard up, sometimes a small batch of beer is good because it doesn't use up too many bottles and you can brew more variety more frequently.
Over the years I have done lots of two can mixes. Some that come to mind are a Wander's Pilsner (I think) + a Munich lager which tasted great, aged well and satisfied lots of discerning continental beer drinkers I know. I have also done a few "Half and Halfs" by using Cooper's light (real ale) and dark ale together which results in a strongly hopped, fruity tasting ale that also seems to keep very well. Wander's Munich Lager makes a pretty impressive 11.5 litre batch too: my girlfriend and I are currently finding it to be very drinkable.
I really recommend the two can method, as it makes for a very pure product which is not as cheap as the can and sugar (or dextrose) method but is cheaper than a hopped can of extract plus an unhopped 1 kg can of plain malt extract. Perhaps another way to reduce the bitterness in a two can brew may be to boil the hell out of one of the cans of extract with water and add the other one after the wort has cooled to around 70C. This would theoretically give you one can with more aroma hops flavour and one that is not as stronly flavoured which may result in a more balanced brew for those who don't like the extra bitterness.
Anyway...sorry about the rant and thanks for the interesting thread..Prost!!!