Maybe if they were installed into separate directories, and then some way mounted chroot'ed so that either one would appear to be on it's "own" root? Ok, that doesn't sound good, but would be interesting to try if I didn't have anything else to do
But yeah, the limit is only for primary partitions. Somebody else knows better than me why exactly primary partitions still exist, but I don't find a sane reason..Windows might get a hiccup if you tried to install it onto a logical partition, but Linux isn't so pecky, it runs just as well on a logical partition than on a primary partition. So if you have two partitions for Windows and one for Fedora, that leaves one extra (your swap?) where you can add several logical partitions - enough for your Gentoo and swap. Of course you can create a partition layout as you wish, but that way you wouldn't have to do much changes except for telling Fedora where the new swap partition is; if you decide to make some other partition the logical one, you need to deal with moving operating systems. A swap partition is more easily created, moved and so on..