The monitor IP must be routed through the tunnel, the endpoint IP mustn't. The tunnel can't be routed through itself.
Who is your VPN Provider? If both tunnels use the same ip config, which I see often with WG, you can not have more than one tunnel to that provider.
Make sure not to use the same monitor IP for both. Ping distinct public addresses instead, like 1.1.1.1 and 1.0.0.1. Using unique gateway IP addresses might be required, too (they're arbitrary anyway). Last, you can remove the /28 from the tunnel addresses to avoid having two interfaces with the same subnet (might not be strictly required though).
Have you tried all of this?
You have to use the tunnel address provided by the VPN provider, correct. But the prefix length and gateway address don't matter. These are only used locally by OPNsense.
[Interface]# Bouncing = 10# NetShield = 2# Moderate NAT = off# NAT-PMP (Port Forwarding) = off# VPN Accelerator = onPrivateKey = ******Address = 10.2.0.3/32DNS = 10.2.0.1[Peer]# UK#53PublicKey = ******AllowedIPs = 0.0.0.0/0Endpoint = 146.70.83.66:51820
So, indeed, I did use those two addresses arbitrarily