First add another network interface to Server1, then connect it to Subnet2.
Virtual network and subnets.
A subnet is a range of IP addresses in the virtual network. You can divide a virtual network into multiple subnets for organization and security. Each NIC in a VM is connected to one subnet in one virtual network. NICs connected to subnets (same or different) within a virtual network can communicate with each other without any extra configuration.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/network-overview