combining two projects into one

Projects General Guidance
2

Put the two things in functions, then implement a basic state machine. So you get something like this:

#define LINEFOLLOWER 0
#define OBSTACLEAVOIDANCE 1
byte mode = LINEFOLLOWER;
void loop() {
  checkIR();
  switch (mode) {
    case LINEFOLLOWER:
      linefollower();
      break;

    case OBSTACLEAVOIDANCE:
      obstacleavoidance();
      break;
  }
}

void checkIR() {
  // Read command over IR.
  // set mode accordingly.
  if (command == 'l') mode = LINEFOLLOWER;
  if (command == 'o') mode = OBSTACLEAVOIDANCE;
}

void linefollower() {
  // original linefollower loop()
}
void obstacleavoidance() {
  // original obstacleavoidance loop()
}