Yes, it is possible to use multiple stings with one gateway. In this case, take 1 DC cable of both and route them through the gateway. Please note that you cannot connect 2 strings on 1 MPPT on your inverter. For 2 strings use 2 seperate MPPT connections on your inverter. E.g. string 1 to MPPT 1 and string 2 to MPPT 2.
For more information please check the quick guide which you can download here.