Per instructions the whole house water filter WHELJ1 is installed in line before the softner WHES44. Question is on the drain line for the recharge - in NV the loop is installed in the garage with the line up 5 feet, over about 10 and down into the laundry room drain. Can both recharge lines be put into Tee to the single drain line by the loop in the garage - when one is recharging will it backwash into the other unit? Do I need to add check valves to prevent this? I called whirlpool and they told me no but find it hard to believe.

Thanks

JB

Manufactures installation instructions specifically says not to tie them together.
As long as the filter and softner do not discharge at the same time, it will work. Check valves will not help because as long as the machine is not discharging there's a closed solenoid valve that will prevent any back flow, and if it's discharging the check valve will be open.

One issue is if the are both running to drain at the same time, it could increase static pressure in the draining line, creating the possibility of an incomplete rinse of the softener or filter... Do the extra work to keep yourself healthy.