A single big battery will discharge slower and you may not draw it down as much.
So in theory a single battery will last longer.
But if you can get more amp-hours for a cheaper cost then two batteries are probably the way to go.
Also it depends on where you're going to mount them.
You want to keep the trikes CoG as low as possible, therefore its more stable on high speed corners.
So if you need to mount a single big battery high up vs two smaller ones lower down, then how brave are you ........