You have gotten some very good battery recommendations here based on years of ES member experience. These estimates can be a little elusive for the beginner so here's a bit of a battery SWAG (Scientific Wild Ass Guess) that might be helpful to fill in the blanks and let you jiggle numbers a bit yourself to do some 'what ifs'.
Starting with a projected average power consumption: 25Wh/mi is not unreasonable for what is being attempted and allowing for small hills, although somewhat lower might be required for a dead flat terrain + increased pedaling, and (obviously) considerably more for a frisky wrist on the throttle.,
So - using 25Whr/mi as a basis, assuming a 20% margin for battery deterioration over time and perhaps cold weather running, and using an 80% depth of discharge for good LiFePO4 life, we get an estimated energy requirement for your pack:
25Whr/mi x 28 mi x 1.20 (%safety margin) / 0.80 (%DOD) = 1050 Wh
We can get this much energy on board by trading volts and amps to dial in an effective packaging/cost solution. Trying out different packs with 12s or 16s configurations and assuming a mean cell voltage 3.2v over an 80% LiPO4 discharge cycle, we get:
for 12s:
1050 Wh / (12s x 3.2v/cell) = 1050 Wh / 38.4 v = 27.34 Ah (buy two 36v 15Ah packs)
for 16s:
1050 Wh / (16s x 3.2v/cell) = 1050 Wh / 51.2 v = 20.50 Ah (buy one 48v 20Ah pack or two 24v 20Ah packs)
Choose the packs based on packaging, etc. With a cargo bike the availability of two rear sideloaders makes the dual 24v packs attractive as a quickie solution. The higher voltage of the 24v packs will give you better speed options, a simple series connection, and so looks like a good choice. On the other hand, a 72v DD solution could be approached with the dual 36v 15Ah packs in series, again giving you the required 1050 Wh of capacity.
Here is a quick recalculation dropping the cold weather/aging safety margin and assuming a slightly lower consumption for pretty flat terrain:
21Whr/mi x 28 mi x / 0.80 (%DOD) = 735 Wh (very similar to Dr Bass estimate)
for 12s:
735 Wh / (12s x 3.2v/cell) = 735 Wh / 38.4 v = 19.14 Ah (buy one 36v 20Ah pack)
for 16s:
735 Wh / (16s x 3.2v/cell) = 735 Wh / 51.2 v = 14.35 Ah (buy one 48v 15Ah pack or two 24v 15Ah packs)
Anyhow - not much new here for recommendations, but maybe this will help you wrap your head around the advice here
As far as 2WD goes: remember that both motors need not be running at all times and are generally not needed on the flat. If you are concerned about redundancy, a small geared hub as a backup could be added that would normally not be in use. This would be primarily for a low powered 'limp home mode' and could be priced appropriately and added later in a phased build. The freewheel of a small gear motor will incur no drag penalty when you are running on the other motor. In any case, your best battery setup for two motors will be to share a common battery to optimize access to whatever power you are carrying along. I'm not pushing for a 2WD solution, but encouraging you to think creatively to fulfill your needs.
