I wouldn't set it up to use columns in the first place. I'd just set up SubReport2 in the same section as SubReport1 and Subreport4 in the same section as Subreport3 - you can put them side-by-side with no issues. Then put Subreport5 in its own section and make it as wide as both the subreports above it. The only formatting thing that might appear is if one of either pair of subreports is longer than the other then there will be blank space under the shorter one. There is no way around this in order to get the format you're looking for.
-Dell