Given that the prospect of West coming up with a helicopter significantly larger than a CH-53 or a CH-47 in the near future is most unlikely, and the West's relationship with Russia, home of the Mi-26, could be frosty for some time to come, the maximum capacity for heavy lifting is going to remain more or less static for the foreseeable future.
Using more than one helicopter on a single load immediately multiplies the lifting capacity, and has been done with small helicopters (MD-500s (see video from 01:45)), so is it feasible with the West's heavy lifters??
A bit of Googling came up with a some videos, technical reports etc. but nothing to suggest actual experiments with large helicopters.
1971 proposal by Piasecki to join two CH-53s (not a multi-helicopter lift, more of a new design):
http://www.dtic.mil/dtic/tr/fulltext/u2/743516.pdf
Paper comparing dual lifts with and without a spreader bar (includes trial with two R-MAX unmanned helicopters):
http://uarc.ucsc.edu/flight-control/...14_Berrios.pdf
What additional problems would there be with larger helicopters, and is two the limit? If the lifters were unmanned, would swarming software be appropriate to control them?