One-way and two-way sync between Shopify stores
Two-way sounds like the better deal: edit in either store and both stay current. It is the better deal right up to the first minute when the two stores disagree about the same field, and that minute always arrives.
The conflict problem in one paragraph
At 10:00 someone fixes a typo in the description in store A. At 10:01 someone rewrites the same description in store B. Both changes are real and both are wanted. A two-way sync has to pick one, and every rule it can pick by is wrong sometimes: last write wins throws away the careful edit because it happened first, field-level merging produces a description neither person wrote, and asking the merchant every time turns the app into a second inbox.
One-way sync sidesteps this by declaring which store owns the catalogue. There is no conflict because there is no second opinion.
What one-way actually means in practice
The store that owns the catalogue is never modified by the app. Edits there travel to the receiving store within seconds. Edits made in the receiving store are noticed, reported as differences, and - if syncing is on - replaced by the value from the owner on the next check.
That last part is the one to think about before you install anything. If your team edits products in the receiving store on purpose, one-way sync will keep undoing their work, and correctly so: you told it who owns the catalogue. The answer is either to edit in the owning store, or to sync only the products that are genuinely shared.
Where two-way is genuinely worth it
Stock, in one specific shape: several stores selling from one shared pool, where a sale anywhere has to reduce availability everywhere. That is not really two-way editing, it is one shared number with many readers, and it is a different product from catalogue sync.
If that is your case, look for an app that says explicitly how it resolves two sales in the same second, and test it before you trust it with a season.
What StoreTwin does
One way, by design, with the direction chosen when you pair the stores: the pairing code is created in the store that owns the catalogue and entered in the store that receives it. A store can receive from several sources, but a single product in a receiving store belongs to one source only, and pairing stores into a ring is refused outright.
If the receiving store needs to keep its own values for certain products, sync a subset: choose the products to sync, and the rest are still audited and reported without being copied.