So this is how the conversation went:
Me: Steve, we don't need two carts, we just need a few things for KTs birthday.
Steve: Uh, I think we really should get two.
Me: Nah, we'll just put our few items around his car seat.
***20 minutes later***
Me: Hm. I think we could have used two carts.

Notice the eggs (for her birthday cake), the cake mix, the toys. Well, at the very least Matthew did not care at all. :)
Next time, we'll get two carts!