Strange thing? We already stack the Earth. Not quite as grandiosely (is that really a word??) as you're asking for, but when you think about it modern cities, especially those with skyscrapers, are attempts to do exactly what you're doing.
So rather than getting the world engaged in a megaproject to build a series of 100km high shells, instead get the world to work towards covering the surface in high density urban environments. Even if your skyscrapers have each floor the height of ten, you can cram an awful lot of usable surface area into it. If you use the same amount of mass as you'd need for even one mega-shell purely to build cities you'll be able to eke out a lot more usable surface area.
This neatly sidesteps the issue of varying g and of needing unobtanium at the same time. Human superstructures, no matter how tall, won't take you high enough to get to 0.5g unless you use unobtanium. It does however raise a whole new slew of issues, but there are none that we haven't dealt with before:
First up: Food. Getting food into and out of the city is going to be impossible if you want to maximise the surface area, as you'll have a world-spanning Coruscantian conurbation. Instead: Grow the food in stacks. Vertical farming is beginning to take off as a science, with a combination of hydroponics, high quality nutrient feeds and solar-mimicking LED's, you can stack not only the earth, but also the fruits of the earth.
Secondly: Power. Power generation is going to be tricky, as people don't tend to like living directly on top of power stations, and you don't want the chance of a nuclear reactor melting down onto you. The solution? Cover the top of every building in eco-power solutions, and bury your nuclear reactors deep in geologically stable plates. Either that or build power generation districts where all the buildings are dedicated to generating power.
Thirdly: Water, sewage and transport. The plumbing will have to be IMMENSE. The easiest way to get round it is by localising the water systems to a certain radius and having municipal water processing buildings that take sewage in at the bottom, pump water out of the sides, water vapour out of the top and high-nutrient food blocks to the nearby farm towers.
The thing is that all these issues are logistical ones. OK, they're logistical nightmares, but so is the logistics for shifting a fraction of the Earth's mass into a free-floating shell!