You don't want the front section tuned lower. The front section acts as an acoustic low pass filter, so if you tune the front lower than the rear you won't have much output.
It's actually the low-pass effect of the front port that let's you use smaller ports in the rear chamber, as the chuffing is masked.
If you have 5 ft^3, do about 1-1.5 in the rear with about 15 in^2 of port It's still going to be a huge port though, probably ~40" in length if you tuning in the high 20's. However, that's still less than 0.5 ft^3 of port volume which isn't terrible, just a PITA to make as you'll probably have to fold it. You can make the rear port external to the rear chamber so that it takes up more space in the front chamber. It's far more important to get your rear chamber built correctly and not too small rather than worry about gain from a high ratio enclosure for listening to music. After that use whatever space you have leftover in the front chamber for your higher tuning. This is assuming a single driver and a series bandpass configuration.