the servo will keep running at max speed.
lspeed = startSpeed;
rspeed = startSpeed;
leftServo.write(lspeed);
rightServo.write(rspeed);
That is what you ask for.
You have to asses that the bot is in the right direction at all times. Both servos should never go at full speed unless the light is straight ahead.
Jacques
EDIT: The ratio of light recieved by the left & right sensors should be proportional to the speed at which the left and right servos turn at all times. (Or inversley proportional if the freewheel is at the back)