Two dogs on one pad is not twice the problem. It is a different problem, because the second dog changes what the first one does with the spot.
The pad stops being neutral
A pad used by one dog is a place. A pad used by two is a place with a message on it, and dogs read each other's marks whether or not either of them is a marker in the usual sense.
That is why a setup that worked perfectly with one dog can start producing misses, or a sudden interest in the same patch of floor, the week a second dog arrives. Nothing has gone wrong with the training. The meaning of the object changed.
One pad or two
The honest answer is that it depends on the dogs, and that you can find out cheaply rather than guessing.
One pad is often enough for two dogs who already share space easily, eat near each other without tension, and are similar in size. Two pads are worth it when either dog hesitates while the other is nearby, when one is much larger, or when one is a puppy and the other an adult, because their schedules will not line up.
The test is simple: watch whether either dog waits, circles longer than usual, or leaves without going when the other is in the room. Any of those is the dog telling you one pad is doing two jobs.
Two pads in one room, well apart rather than side by side, a dog settled near each, seen from standing height.
Placement matters more with two
With one dog you can put a pad in a corner and stop thinking about it. With two, the corner can become a trap: a dog that gets approached while going has nowhere to move.
If you run two pads, put them apart rather than side by side. Two pads touching are one large pad as far as the dogs are concerned, and the whole point is that each has somewhere that is not the other's. Away from food and bedding still applies to both, and the reasoning is in why a dog misses the pad.
Washing changes shape too
This is the part people underestimate. Two dogs means the pad reaches the point of needing a wash sooner, and the smell timeline runs faster because there is more on it.
A litter is this problem at its limit, and whelping and recovery is where the wash load stops being occasional. The rotation that works for one dog with a 2-pack often does not stretch to two dogs, and the fix is more pads in circulation rather than leaving one down longer. Leaving it longer is the one response that makes the smell problem worse, for the reasons in why the smell gets worse after a few days.
A puppy joining an adult
The common case, and the one with the clearest answer. Give the puppy its own pad, in its own area, and do not expect it to share.
An adult dog's mark on a pad is not a welcome sign to a puppy still working out the rules, and a puppy's accidents near an adult's spot can unsettle a dog that was previously reliable. Separate places, separate schedules, for as long as the puppy is still on a puppy's timetable.
When one dog starts marking the pad
Marking over another dog's spot is a different behaviour from going, and it usually shows up as small amounts, often at the edge, sometimes vertically against something near it.
A larger pad does not fix it, because the dog is not missing. What tends to help is separating the places, cleaning thoroughly enough that the previous mark is genuinely gone, and, if it persists, a trainer. We say the same thing about male dogs who lift and aim off the pad: no size we make solves it, and a raised edge or tray is a different product.
The honest limit
We sell pads in packs of two, so "you may need more" is a sentence we benefit from. Which is exactly why it is worth being precise: plenty of two-dog households run fine on one pad and one spare, and the ones that do not usually know within a fortnight.
Watch first, buy second. If one pad and one spare is holding, it is holding.
When it is worth a vet's opinion
A dog that was clean and starts going indoors when a second dog arrives is usually reacting to the change, and that is behavioural. But sudden onset, straining, going far more often or drinking noticeably more is worth an opinion regardless of what else changed in the house, because a coincidence of timing is easy to mistake for a cause. What shifts normal frequency is in how often a dog needs to go.
Questions people ask us
Will two dogs use the same pad?
Many will. Whether yours do is answerable in a week of watching, and the signals are hesitation, longer circling, or leaving the room without going.
Do I need a bigger pad for two dogs, or two pads?
Two pads, in most cases. A bigger pad gives one dog more room; it does not give the second dog a place of its own, which is usually what is actually missing.
My older dog started having accidents when the puppy arrived. Why?
Usually the change rather than the puppy. Dogs regress under household change, and the fix is consistency and time rather than equipment. If it persists past a few weeks, it is worth an opinion.
Should I wash both pads together?
Yes, and the settings do not change: warm below 40C, no softener, tumble low. Two dogs means more frequent washing, not a different method. The care guide covers it.
Does one dog's smell on the pad put the other off?
Sometimes, and it is the most common reason a shared pad stops working. If that is what you are seeing, separate places will do more than a stronger cleaner.