HI davorin,
there is no license limitations on site-to-site VPN connections. The license limitation is regarding VPN clients (dynamic vpn). You get 2 concurrent connections included in the base system and can purchase extra if needed. In your setup I don't expect that to be a problem :-)
You can possible use openvpn on Linux to emulate a site-to-site connection and Pulse Secure (the client used for dynamic VPN) can be found in a Linux version.
I hope this answers your questions.