Whether a backup may share discharge piping depends on the exact pump design and approved installation. Zoeller's 508 FIT battery system includes a combined tee and check-valve arrangement, while its 540 FLEX water-powered manual requires a separate discharge. Do not copy one layout onto another. Have the installer verify valves, capacity, outlet approval and what a shared blockage would affect.
Use the manual for the actual backup type
A second pump does not automatically need the same pipe arrangement as the first. Identify the primary pump, backup pump and supplied installation kit before choosing a shared discharge. This answer compares specific Zoeller battery and water-powered systems with Liberty's SJ10; it does not approve a generic tee for every sump.
The Zoeller 508 FIT battery-backup manual, FM3120 Revision G, supplies a documented combined arrangement. It incorporates a check valve at the backup discharge, specifies a check-valve assembly on the primary pump and joins the discharge through its kit's tee. Remaining piping continues above that tee.
Have the installer use the complete compatible layout and instructions. A fitting that physically accepts both pipes does not establish that the valves, seals, float clearances or discharge performance are correct.
Do not transfer that layout to a water-powered backup
Zoeller's 540 FLEX instructions say its water-powered discharge should not connect to the primary sump discharge and specify separate piping and an air-gap arrangement. Liberty's SJ10 manual independently says not to connect its discharge to the primary sump pump or directly to the municipal drain.
Those are actual manufacturer installation directions. Their references to plumbing rules are not a verified permission or prohibition for every different product and address. Have the installer confirm the selected system and the authority's applicable outlet, backflow and air-gap requirements before altering pipes.
Ask what a common blockage would disable
A battery provides another energy source. It does not provide another route around a blockage in a common downstream pipe. A backup connected to that blocked route can still be unable to remove water.
Discuss the actual discharge height, restrictions, outlet condition and intended operation with the installer. Pumping capability changes with installation conditions. Do not combine advertised pump ratings or assume a pipe is adequate solely because it worked with the old pump.
Check-valve and repeated-cycling symptoms need their own investigation. Do not remove a check valve to make both pumps move water or reach into an energized pit to test a float.
Verify both layout and destination
Approved discharge destinations are a separate question from how two pumps join. A correct kit layout does not authorize sending clear groundwater into any convenient drain, and sewage equipment requires different handling.
Keep the installation diagram and selected manuals with the records. Have a qualified provider verify both pumps using the manufacturer's complete procedure, including free float movement and accessible service components. Backup selection and ongoing maintenance remain necessary even when the piping arrangement is approved. A dry pit or a quiet alarm alone does not establish that either pump can discharge under the conditions it must handle.