To have 2 stop bits in RTU communication it is needed to use "No Parity" settings. If the parity is set to ODD or EVEN then the communication use 1 stop bit.
The number of stop bits is taken according the parity settings. This is made according the Modbus over serial line specification.
The standard Modbus RTU with 11-bit format including 1 start bit, 8 data bits, 1 bit for parity, and 1 stop bit. If no parity is implemented, it will require 2 stop bits for 11-bit format though.