We went with two single mattresses on one frame and have done for four years. Honest report, since you asked about the trench.
The gap is real and it is manageable. Two mattresses of exactly the same height, pushed tight, with a fitted sheet across both or a gap filler strip. You feel it if you lie right on the join; you do not notice it otherwise. The height matching matters more than anything: a centimetre of difference is far more annoying than the gap itself.
What it is genuinely good for: each person gets the support they need, motion transfer between sides is essentially zero, and you can replace one side when it wears without replacing both. That last point is worth more than people expect.
What it costs: the join, and the fact that a standard double is two narrow singles rather than two comfortable ones. If you can go up a size in the frame, do.
On adjustable beds with separate sides: they work and people who have them generally like them. They are expensive, heavy, and they add mechanical parts to a thing that otherwise has none. If the budget is there and you want to try firmness levels without committing, they are a reasonable answer. I would not buy one to solve a difference that two mattresses solves for less.