I assume that you are referring to what is called bonding, a feature, which allows to combine bandwidth provided by multiple ISPs. Without third party support, Bondix in Teltonika case, which is also available for certain devices and firmware only, this is not possible.
What is supported, is called Load balancing. In this case, one of the RUT240s could be used as a secondary wired WAN source for the other. This combination allows to distribute traffic between two providers networks, meaning that it will not increase the speed of a single connection, but the number possible of simultaneous connections will. Also stability and reliability of your network will improve too.
For a more detailed explanation, please refer to this page.