Backup Sump Pump Weep Hole

I have a backup pump connected to a battery backup in with my main sump pump. Each pump has a separate drain line coming out of the basin and about 3 feet above the basin, a Y connector joins the lines together. Just prior to joining, each line has its own check valve. My backup pump recently just died as indicated by the battery backup system so I am planning to replace it. I have only been in this house for about a year and due to radon, the pump basin has been sealed up. I unsealed it to take a look at the backup pump to get a replacement. That's when I was introduced to the weep (relief) hole. I was surprised to see water shooting out of the hole back into the basin when the pump kicked on. After a quick search I understood its purpose and that it was supposed to be there. Here is what I don't understand. The hole is drilled into the line coming out of the backup pump. There is no hole in the line for the main pump. When the main pump kicks on, water sprays out of the hole even though the backup pump is not running. How is this possible? Is that the intent? The only thing I can come up with is that the check valve on the backup line is faulty. When the main pump is working and moving water, some of that water shoots off into the backup line, past the faulty check valve and sprays out of the weep hole. Since I am going to be replacing the backup pump, I want to make sure I am understanding how it is supposed to be setup. Thank you.

 

I always preferred using two drain lines to outside and then into a common line.
From your description..... the aux sump pump check valve is not fully closing OR is being drawn open by the main pump. It shouldn't really be a problem with only a small bit of water coming back.

The weep hole in the line above the aux pump is to allow the pump to start as its not under pressure. Your main pump may have one under the pump or may not need it.