I don't think you can have 2 gateways on 1 server, you can only have 1 gateway.
Just setup the local IPs on your second NIC config on these servers and if they're connected to the same local switch they should be able to connect with each other.
I've run hundreds of FreeBSD servers with this setup. When doing this I separate the public switch and the local switch. Not sure if you can do this if you're doing dedicated server with a provider.
██ We Have Generated Over 7 Million cPanel Backups Come Dance Together With Us Y'all!
██ Offer Your Own Backup Hosting Service - SiteAutoBackup.Com (Private Label / WHMCS Ready) ██ WebHostingBusinessBook.Com | YouTube.com/WebHostingTutorial | NowOpenOnline.Com