Two cats fighting over one scratching post

Two cats sharing one scratching post is a recipe for exactly what you're describing — resource guarding, tension, and one cat (usually the younger one) getting pushed out. The most common reasons cats ignore or fight over posts are that they're too short for a full stretch, too wobbly to feel safe, or the wrong material. Before I point you toward a fix, a couple of quick things: roughly how big is your Ragdoll, do either of them scratch horizontally as well as vertically, and what surfaces are they going for when they're not on the post?

That's a useful combination — a big cat who needs serious height and stability, a younger cat who's targeting fabric furniture, and a stair carpet scratcher which is a classic horizontal-ish scratching behaviour even if they're reaching up. The stair thing tells me your Ragdoll might actually appreciate a sisal flat pad or angled scratcher alongside a tall post, but the sofa corner is definitely a vertical sisal job. What's your current post made of and roughly how tall is it?

There's your problem, honestly. 45cm is too short for a Ragdoll to get a proper full-body stretch — they need at least 90cm ideally — and carpet covering is often the wrong material. Cats scratch to shed claw sheaths and mark territory, and sisal rope gives them the resistance and texture they're actually looking for. Carpet can feel too similar to the floors they walk on, so it doesn't satisfy the urge the same way. You also only have one, which means whoever gets there first owns it.

In most cases, yes — two posts in different locations removes the competition entirely. The rule of thumb with multi-cat households is one resource per cat plus one extra, and that applies to scratching posts as much as litter boxes. My specific recommendation for you: get two of the Feandrea 150cm Cat Tree with Sisal Scratching Posts. It's tall enough for your Ragdoll to fully extend, the sisal rope is the right material, and the wide heavy base means it won't wobble when either cat hits it hard. Put one near the sofa where the Siamese is already scratching, and one near the stairs for the Ragdoll. Placing it right where they're currently scratching is the fastest way to redirect them — you're not asking them to go find a new spot, you're replacing the bad target with a better one in the same place.

Not overkill at all — Siamese are lean but they're athletic and they like to climb and stretch. A taller post gives the Siamese more options, not fewer, and having platforms at height will appeal to the Siamese personality specifically since they tend to like being up high and surveying the room. You might find the Siamese uses it more than the Ragdoll once it's in place. The main thing is that both cats get their own post in their preferred spot, so there's nothing left to fight over.