We do it
I know it's not really safe
Usually at Kroger we go for one of the carts with a car attached. And sooner or later there is arguing, so we move one kid to the front "baby seat" (both kids are around 40 lbs)
But Walmart doesn't have car carts - only those big, molded seats at the back of a regular cart. And those promote kicking one another for amusement.
So the alternative is one in front, one in back. Or one walking, which soon turns to complaining, running in front of the cart, or hanging off the side of the cart, which I consider more dangerous (for tipping purposes).
Whoever sits in back must stay SEATED. FWIW, I was a kid who fell out of the shopping cart at age 3 -- one of those very high, very shallow kind (what was my mother thinking?!) I hit face first, lost consciousness, blacked both eyes ... daycare called CPS because of the black eyes and my refusal to give a plausible reason why ... yet I still do put a kid in the back of a (regular, deep) cart. And stay right beside the cart.
Shopping with 2 kids - it's not fun.