In my experience, trying to juggle one Walmart account between two people is a disaster waiting to happen. I tried doing this with my brother last year to split bulk costs and the app definitely booted one of us out every single time we tried to add an item simultaneously. It basically just overrides the cart contents depending on who hits save last. Here is how I finally fixed it:

• Build two separate lists in the notes app or a shared doc first.
• Have one person place the order.
• Use Venmo or CashApp to settle up the difference. It is way less stressful than fighting with the Walmart app and losing your progress halfway through. Honestly, dont even bother trying to log in at the same time. Just a heads up, Easy Cart Share is really helpful for sending specific Walmart lists to family members.