Configure Multiple NAT gateways

Question

I have two public & private subnets across two AZs. I want to create a NAT in both public subnets. How do I route my private route table to use both NATs? Case three in this accepted answer. I know it's possible but I'm unsure on how to implement it. Should the destination cidr_block be the public subnet's cidr block? I also tried 0.0.0.0/0 and errored out.

resource "aws_route" "r" {
  count                     = length(aws_nat_gateway.this)
  route_table_id            = aws_vpc.vpc.default_route_table_id
  destination_cidr_block    = "aws_subnet.public_subnets.${count.index}.cidr_block"
  nat_gateway_id            = "aws_nat_gateway.this.${count.index}.id"
  depends_on                = [aws_vpc.vpc,aws_subnet.public_subnets,aws_nat_gateway.this]
}

When I tried 0.0.0.0/0

error creating Route in Route Table (rtb-059eee10e310ade77) with destination (0.0.0.0/0): RouteAlreadyExists: The route identified by 0.0.0.0/0 already exists.