No… these need to be in the same entry or the one will override the other. Just separate them with a space…
EXTRA_JAVA_OPTS="-Dgnu.io.rxtx.SerialPorts=/dev/ttyUSB0:/dev/ttyS0:/dev/ttyS2:/dev/ttyACM0:/dev/ttyAMA0:/dev/ttyUSB1:/dev/ttySlimmeUSB:/dev/ttyTeleUSB -Xms250m -Xmx350m"
You will need to restart OH to apply the changes.