Will it be watery?

I was thinking about fermenting a strong ale using a 1.8kg can of hopped LME and a 1.8kg can of unhopped LME and 2kg of dextrose with some high alcohol yeast to make a 20L batch. Will it taste a bit thin because of the dextrose or will that amount of LME (3.6kg total) give it enough body?

Why not ferment the LME first, then add the dextrose a bit at a time; or even split the batch using different amounts of dextrose?

I've never figured out what people mean by watery.

That's a lot of dextrose! I'll sometimes add dextrose to a dry stout that already has some fermentables. The maximum I do is 1 lb, or .5 kg. With your current plans, it sounds like you'll be making some "prohibition beer". What Americans made during prohibition: when they used equal portions of malt extract with sugar. From what I hear, it's not as good as a pure ale...but it's still drinkable.

+1 to adding the dextrose to the fermenter.

It only takes about a day for actively fermenting yeast to convert a pound of sugar to alcohol and CO2. Near the end if your beers initial fermentation take a hydrometer reading. note this and add 500g of dextrose. Wait a day or two and repeat. Taste the hydrometer sample too make sure you aren't getting that thin taste you are worried about.

Or should I just buy another can of LME and so I'll have 3X 1.8kg cans of LME with 1kg dextrose?