One of us wants firm and one wants soft, what actually works, short of two beds?

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.

Before choosing a solution, separate two things that get confused and that changes what you need.

Support is whether your spine stays in a neutral line. That is a function of your weight, your build and how you sleep, and getting it wrong is what produces aches.

Feel is how the surface behaves at the top, plush, springy, sinking. That is preference.

A lot of firm-versus-soft disagreements are actually a support difference wearing a preference costume. Two people of noticeably different weight need genuinely different support to end up in the same spinal position, which is why a medium fails both - it is not a compromise, it is the wrong support for both of them in opposite directions.

Why that matters: feel can be adjusted on top of a mattress; support largely cannot. A topper changes the surface. It does not stop a heavier person sinking too far into something too soft for them.

So the question to answer first is whether your difference is weight-driven or taste-driven:

• Similar weights, different taste: a topper on one side works well and is the cheap answer.
• Meaningfully different weights: you need different support, and that means either a split setup or a bed with genuinely independent sides.

The cheap experiment that I would do before spending anything, because it answers the question for a fraction of the price: a topper on one side of what you already own.

Buy one topper, put it on the side of whoever wants softer, and live with it for a month. A fitted sheet holds it in place well enough.

What you learn:

Whether the softer person is fixed by a surface change alone. If yes, you are done and it cost very little.

Whether the firmer person was actually fine on the current mattress and it was only ever the other person's problem.

What direction to go if you do end up replacing things.

It looks slightly odd and nobody sees your bed.

Two things worth knowing if you go on to buy anything:

Use the trial period properly. Most mattresses sold with a trial need weeks before your body adapts, and people return them at day ten. Give it the full period, and mark the date you need to decide by.

Buy for the heavier sleeper if you must have one mattress. Somebody too light for a mattress is uncomfortable; somebody too heavy for one is unsupported, which is the worse failure.