How does a network work, and how can you bridge the internal network with the external network?
The answer to the above question is the answer to your problem.
Your ISP gives you 1 public IP Address. This IP Address links to your modem, which then links to your router. The router forms your internal network and through Port Forwarding, it allows connections from the internet to be routed to devices in your network.
When you have a DDNS service in the mix, all it does is translate an FQDN (Fully qualified domain name) into an IP address. You have 2 Raspberry Pi devices in your local network, and even though you use 2 DDNS services, it is the router that determines how the network traffic works. So both DDNS services register the same IP Address to both FQDN'. And this is totally logical and acceptable.
The only thing you need to do, is set the portforwarding correctly on your router to forward the VPN traffic to your VPN server.
Here's an example overview of how your situation could look like.
Your ISP's public IP address is: 123.45.67.89
DDNS1: server1.domain.net points to 123.45.67.89
DDNS2: server2.domain.net points to 123.45.67.89
Router configuration:
Raspberry Pi webserver IP: 192.198.0.11
Raspberry Pi VPNserver IP: 192.168.0.12
Port forward: 80 -> 192.168.0.11
Port forward: 443 -> 192.168.0.11
Port forward: 1723 -> 192.168.0.12
Allow Protocol: 47 (PPTP)
As you can see, both DDNS are the same, just the name is different. You could merge them into one, but if you plan to move one of the Raspberry Pi's to a place outside your current network, I suggest to keep them in check.
Regardless on where you are, the router needs portforwarding to make sure the data reaches the correct server.
Lastly, do note: many routers do not allow loopback traffic. If you try to connect to the DDNS address from within the network, it will likely somehow fail.
