Are you really worried about the backpacks being outside? These tents still have a "cover" over the vestibule. It won't keep bugs out of your bags, but will definitely keep them dry.
My brother has one of these (a Macpac Olympus) :
http://www.macpac.co.nz/catalogue/pr...9-2f661f9fcc79
The vestibule area is quite sizable and easily has room for two packs and inside it is still spacious enough for two thermarests.
Personally, I have a dome tent which is 3 person size (my wife and I have this huge inflatable mattress that easily fits inside - but it is not a 'name' brand tent and I wouldn't recommend it for hiking - we did lug both tents with us around California, Alaska, Idaho and Nevada a few years ago (we really only took mine because my brother had a friend join us for the Alaska part of our vacation...)
Anyway - if weight is an option, I'd recommend the 2 person ones, if you can get over the "bags outside" issue.
Looking at the macpac site, maybe something like this (Macpac Citadel):
http://www.macpac.co.nz/catalogue/pr...8-b3e5bbb090d1
This is only 3.7kg (the olympus is 3.4).
The macpac site has a retail locator too:
http://www.macpac.co.nz/public/retail_locator/
Be warned - macpac ain't cheap - Paddy Pallin (one of the bigger outdoor retailers in Oz) has the citadel listed as $949 AUD