The big issue you are hitting is that Policy Based VPNs can only have 1 connection. Your best bet for this setup is to change your existing gateway to a Route-Based Gateway and use 'PolicyBasedTrafficSelectors' to connect to multiple policy-based VPNs. You will likely have to re-establish your existing S2S VPN when you make this change.
A Virtual Network can only have one VPN Gateway, so you will not be able to make a new Gateway unless you also make a new Virtual Network. It might be possible to make a new Virtual Network for your new S2S connection, and then connect those 2 VNETs via peering, but the your best bet is to use the method described in the doc above.