I have 2 AZs on my VPC. A public and a private subnet in each of them. Currently I have my application deployed on the private subnets and outbound traffic routed using a NAT Gateway on the public subnets of each AZ. The thing is that some vendors have to whitelist my application's IP. Currently I have 2 IPs, one for each NAT Gateway. Is there a way to use the same IP for both of them without losing AZ redundancy so vendor's only need to whitelist a single IP?
No, as each EIP can only be associated with a single network interface.
The following rules apply: An Elastic IP address can be associated with a single instance or network interface at a time.
See https://docs.aws.amazon.com/vpc/latest/userguide/vpc-eips.html