Sharing a pipe is only viable if the connection and drainage are designed for the appliances intended. A two-port adapter does not guarantee capacity, venting, or backflow protection.
What the installer must check
The flow rate must be evaluated when the appliances discharge together, along with the run to the drain, the function of the trap, and the ventilation required. Water temperature, approved fittings, and access for cleaning or service must also be considered.
A system that works with one appliance can overflow when another empties at the same time. The point nearest the puddle does not necessarily identify where the restriction is.
Intended fittings and prepared connections
The outlets must be designed for the hose and the fastening method. Some fittings are supplied capped; only the factory cap corresponding to the outlet being used should be removed, and using its proper procedure. Unknown parts should not be drilled to create a branch connection.
The hose needs securing and a routing compatible with the appliance drain setup . Pipe diameters or slopes from another installation are not used as a universal rule.
What to watch for in the combined test
The installer must verify discharge without overflow, no backflow to the other appliance, and accessible joints. Later leaks and odors require the cause to be assessed; they should not be hidden by sealing vents or adding a check valve without testing the system.
If water appears in the washer when using the sink, see water backflow after drainage. The timing may provide information, but it does not confirm which fitting is incorrectly installed.
Validate the whole setup under simultaneous demand; the fact that a single discharge works does not prove it has enough capacity to be shared.