Hi Philippe,
It will ultimately depend upon the average power/current draw from both motors combined, but this will ultimately depend upon many factors including the wheel diameter, rolling resistance, reduction gearing, terrain, inclines and the total weight of the vehicle and occupants etc..
If you take a look at the
dynamometer figures for the 3kW motor we can estimate that if each motor had to produce the full 3kW power output it would require a 48V battery capable of delivering at least 136 Amps continuous
(68 Amps per motor).
To provide this amount of continuous power over a two hour period from a single battery would require a capacity of at least 272Ah
(or two batteries, each with a minimum of 136Ah capacity each).
However, your vehicle is obviously not going to be pulling maximum Amps continuously for the whole two hour period, therefore the actual battery capacity can be a lot lower.
If you vehicle only required a combined 3kW of power to climb the steepest encountered inclines with a full load, the capacity of a single battery could be reduced to 136Ah.
If we also assume that the vehicle will spend as much time going downhill as is does going uphill, this could further reduce the total power requirement by approximately 50%
(Requiring just a 68Ah battery).
However, if the vehicle is used on mainly level terrain, it will not require much power just to maintain a constant low speed, and higher power would only be required to get it moving and accelerating up to its typical constant speed.
Unfortunately, until you actually test the vehicle and measure its actual power consumption under typical use, it is difficult to say what capacity battery you will actually require.
Ideally, your battery should have at least ~25% more capacity than you actually need, so that you would typically be using ~80% of the battery capacity, as this will give a longer working life for the battery, and you should still be able to manage a full two hours of use even when the battery capacity has degraded to 80% of its original capacity.
I hope this all makes sense.
Alan 
