[edit - Denny posted his while I typed mine up. I gave some examples and walked through the calcs. Hope it makes sense]
The short answer is yes, and you'll get a few points extra gravity by doing so.
The long answer starts with the question: how accurately must you stick to the recipe?
Converting between dry and liquid malt extract is pretty easy but takes a little math.
DME on average (it varies a little from brand to brand) gives 44 gravity points per pound per gallon.
LME on average (it varies a little from brand to brand) gives 35 gravity points per pound per gallon.
so, in order to convert LME weight to DME weight, you must adjust the weight so that you add the same number of gravity points.
In your case, 7 lbs of LME total, minus 6.6 you get from 2 cans, leaves .4 lbs of LME to replace with DME.
So, to convert back and forth:
Wdme = (Wlme*35)/44
Wlme = (Wdme*44)/35
Wdme = (.4 * 35)/44
Wdme = .318 lbs
In other words, adding .318 lbs of DME to 6.6 lbs of LME is the same as 7 lbs of LME.
By adding a whole pound, you are adding 44*.6 extra gravity points, or 26.4 additional gravity points. divide the 26.4 by your batch volume gives you the number of additional points your OG will go up.
26.4 /5.5 = 4.8 gravity points per gallon or it will roughly add .0048 to your OG assuming a 5.5 gallon batch.
The way to figure out total fermentables in the batch is to multiply the weight of each fermentable by the # gravity points for that fermentable, then add all those up and then divide the total by the batch volume to get the gravity points. Gravity points are then easy to translate into Specific Gravity format by moving the decimal left by 3 and adding 1 :
25 gravity points gives 1.025 SG
48 gravity points gives 1.048 SG
121 gravity points gives 1.121 SG
In this case:
6.6 lbs of LME plus 1 lb of DME in a 5.5 gallon wort:
(6.6*35+1*44) / 5.5 = 50 gravity points giving you a 1.050 OG wort.