Building my first strong ale

I've fermented a few ales from kits in the 4-5% ABV range, but I want something around 8-9% ABV, so should I use 2 cans of extract, maybe 1 can of hopped wheat extract and 1 can of dark hopped extract, 2kg of dextrose and 1 packet of yeast. Or should I buy 2 kits and add them together with 2kg of dextrose and 2 packets of yeast?
I know that adding dextrose instead of DME can lead to a thin taste, however if I'm making a high gravity brew will that be a problem?
I will be using Muntons extracts as it is readily available where I am, and most of those kits and malt extracts have an OG of around 1.040, so if I add 2 kits am I right in saying that the OG should be around 1.080? And what can I expect the FG to be with a high gravity beer?

I would use an alcohol-tolerant yeast instead of dry yeast packets, and make a big starter first.

I did a big extract beer a while back, and it took an exceptionally long time to ferment out. I think it would have been much quicker if I had pitched a lot more yeast.

My beer was over 1.090 and fermented down to around 1.010.

Hi there, you've got a similar idea to what I am going to try. I was originally going to do pretty much the same thing, but I've revised my recipe. I'm doing a black rock IPA, with a can of Black rock amber malt and 1kg dextrose. I figure that it'll be a bit stronger than the orignal recipe, but not too unbalanced, should be about 7%.
I think your first idea is a good one, because if you use 2 kits it'll possibly be too bitter. Given that recipe I think you'll be at around 9%.
Another thing, on yeast it might pay to do a standard kit first, then pitch your big beer onto the yeast cake.

...just make sure whatever yeast you use for the first beer is tolerant enough to handle the ABV of the big beer...