I found perhaps a better solution or sharing two Bela's on Windows. The problem with the current documentation is that you can only share one interface which means only one bela can be on the internet at a time.
My solution is to Bridge the two Network Interfaces, then set one to 192.168.6.3 and update the DHCPD.config and interfaces while to match. Then you can use Internet Connection Sharing in Windows to share the bridge connection with your internet connected network interface. This stackoverflow post was useful https://superuser.com/questions/656998/windows-7-share-internet-connection-with-multiple-network-interfaces.
Also this article was helpful too. I was thinking you could use a bridge between all three interfaces, but it seems you would need to have the Bela's use DHCP to get addresses and wasn't sure how to configure that. https://www.windowscentral.com/how-set-and-manage-network-bridge-connection-windows-10