Action while button is pressed

This statement: data = Serial.read(); consumes the data sent over the serial port, so to get continual movement, you will have to constantly feed in A or B, etc. from whatever is generating that data.

Alternately, you could define a 2nd set of single-character commands to accompany the start-rotation parts so that send one command to turn on the motor, and a different command to turn off the motor. You would need to add a variable to hold the current state (started or stopped) of each motor, and turn on or off those states depending on what commands come in from the user.

(untested code)

#include <AFMotor.h>

//define steppers
AF_Stepper motor1(48, 1);
AF_Stepper motor2(48, 2);

char data;
int8_t motor1state=0;  // 0=> Stopped, -1=>Reverse, 1=>Forward
int8_t motor2state=0;

void setup()
{
  motor1.setSpeed(200);  // 10 rpm
  motor2.setSpeed(200);  // 10 rpm
  Serial.begin(9600);
}

void loop()
{
  Serial.println(data);
  if (Serial.available())
  {
    data = Serial.read();
    if (data == 'A')
    {
      motor1state = 1;
    }
    if (data == 'B')
    {
      motor1state = -1;
    }
    if (data == 'C')
    {
      motor2state = 1;
    }
    if (data == 'D')
    {
      motor2state = -1;
    }
    if (data == 'F')
    {
      motor1state = 0;
    }
    if (data == 'G')
    {
      motor2state = 0;
    }
    if (data == 'E')
    {
    delay(50);
    }
  }

  if (motor1state>0)
  {
    // Step forward
    motor1.step(200, FORWARD, DOUBLE);
  }
  else if (motor1state<0)
  {
    // Step backward
    motor1.step(200, BACKWARD, DOUBLE);
  }
  else
  {
    // Do nothing
  }

  if (motor2state>0)
  {
    // Step forward
    motor2.step(200, FORWARD, SINGLE);
  }
  else if (motor2state<0)
  {
    // Step backward
    motor2.step(200, BACKWARD, SINGLE);
  }
  else
  {
    // Do nothing
  }
}