The Merger is the Splitter in reverse: two belts come in, one leaves. You need it once a factory runs machines side by side, because one hub takes at most 12 belts by a player's count, and every branch you merge frees a port. Since 1.0.4 it alternates between its two inputs, so neither branch is starved. It needs no power.
A Merger brings the sand from parallel Pressure chambers back onto one belt
Where to put a Merger
- Build the parallel machines first, for example two Pressure chambers fed by a Splitter.
- Run each machine's output belt toward the same point.
- Place the Merger there and connect both outputs to its inputs.
- Check its output direction before you lay the shared belt to the hub or the next machine.
Two belts that only look close together are not joined; a crossing raised with Ctrl and the scroll wheel stays separate until a Merger connects it.
Why the merger works for you and not your friend
At launch only the host's mergers worked: one placed by a joining player did nothing, so players let the host build them. Patch 1.0.3 made crewmates' mergers and splitters work, and 1.0.4 let any crewmate place them on belts someone else built. If it still happens, make sure everyone is on the same version, since crewmates must all run 1.0.3 or later to play together.