Just to clarify - is the main question whether Site #1 will be able to reach Site #2? If so, you don’t need to additionally connect from Site #1 to Site #2 if they’re connected to the same Server, which is your Office. Since they’re connected to the same server, they’re technically within the same network, allowing them to reach each other.
The idea is to enable site#1 to communicate directly with site#2 without using the VPN connection via the office, in order to secure the information exchanged between site#1 and site#2.
The minimum requirement is the use of an I/O “RUT956” from site#1 to site#2 directly like remote I/O.
In that case, if I understand correctly, you’d like to use some other VPN option? If so, we have a bunch of configuration examples available on our Wiki, which you can find here: VPN Configuration Examples - Teltonika Networks Wiki
One router shall act as the server, the other - as the client. Regarding I/O, you’d have to elaborate more on what exactly are you trying to achieve, I might be able to explain, or give more information.