This one is probably the quickest and easy, I would recommend it espeically if this is temporary. There should be zero impact on your current setup as there are no changes needed on the PSA itself. The downside is you have to make end users aware of this new hostname incase they find existing one slow
a. create a second DNS hostname for the service example vpn2.myvpn.com such that it resolves to the second public IP hosted by your secondary/less busy circuit
b. setup the NAT rules on this secondary circuit/firewall similar to your existing setup
c. Have end users connect to this new hostname
There are several other ways ot do this, if above does not suit give us more details of your requirements.