When my daughter was born my first thought was to do just that with the front carrier. Then I tried it out and found that its very uncomfortable haveing both straps on at the same time, and almost impossible to (with my front carrier) to clip the buckle in the middle of my back to keep the straps from sliding off. I thought about buying another carrier from a consignment store, cutting the straps off and sewing webbing onto it in place of the straps then connecting cinch buckles to the straps of my backpack and just attaching the carrier directly to the straps of the pack.. then I realised the front carriers would only be good to carry my daughter up to 18 months, after that She would be a pain if I tried to carry her in one (and that held true). Before my estranged wife became estranged I had planed to carry her on alternating shoulders between the stuff on the pack thats behind my head and my head. Not practicle but doable with two people splitting the shoulder ache. I could still do that but I know I would tire quickly, especially as she grows older.
I have been thinking of something like the Sherpani Rumba with 2800 cubic inches and a weight limit of 70lbs it could last a while and, if I don't take a whole lot, has enough storage; But how long will she realy be able to use it.. I've read that by 2-6 years old children will refuse to do the carriers, and by 4 they will probabily be too big for even the Rumba, granted thats 2 years away for me.. but if I am more then likely not going to be able to use it for back country backpacking trips in the next 2 years, because of the estranged wife, is it worth spending 3-4 months savings on? What happens from 4 years to 6 years when they are too big to carry for long periods of times but too small to realy walk any decent distances, even very slowly. Other then relying on some type of cart where I am restricted to streets and very flat domesticated landscapeing, what can be done.

The sling thing looks interesting, you said you used it up to two and a half years old.. how young did you start useing it? How much longer would you say you could see yourself being able to use it?