Second NIC blocks lan access

[yitzyn]

Im running openvpn server on a raspberry pi. the pi has two network interfaces eth0 and eth0.40

eth0 is 192.168.10.2
eth0.40 is 192.168.40.2

the reason for giving the pi a second network interface eth0.40 is because i have airplay devices on the .10 subnet and I'm using avahi-daemon to forward bonjour so that when I'm on my wifi which is my .40 subnet i can airplay to the devices on the .10 subnet.

Everything was working fine before i added the eth0.40 network interface. I was able to access both the .10 and .40 subnet through VPN.

After adding the second network interface eth0.40 I can no longer access the .40 subnet through my vpn only the .10 subnet.

when I shutdown the network interface (ifconfig eth0.40 down) everything works again.

How can i keep both network interfaces up and still be able to access both subnets through vpn.

Any help would be appreciated!!