Simultaneous control of two step motors via usb-serial controller unit from Applied Motion

I run the script under. When I comment out one or the other, of the motors, the script works fine, but when I try with both the script hangs and Matlab goes into a busy mode that no ctrl+c or other kill method can break. Matlab can then not even be closed, nor the OS - I actually have to force-quit with the power button. Suggestions?

s1=serial('/dev/tty.usbserial','Terminator','CR');

s2=serial('/dev/tty.usbserial4','Terminator','CR');

fopen(s1)

fopen(s2)

fprintf(s1, '%s\r', 'HR');

fprintf(s1, '%s\r', 'SK');

fprintf(s2, '%s\r', 'HR');

fprintf(s2, '%s\r', 'SK');

fprintf(s1, '%s\r', 'AC1'); %sets acceleration rate to 1 rev/sec/sec

fprintf(s1, '%s\r', 'DE1'); %sets decelleration rate to 1 rev/sec/sec/sec

fprintf(s2, '%s\r', 'AC1'); %sets acceleration rate to 1 rev/sec/sec

fprintf(s2, '%s\r', 'DE1'); %sets decelleration rate to 1 rev/sec/sec/sec

fprintf(s1, '%s\r', 'VE1'); %velocity to 1 rev/sec

fprintf(s1, '%s\r', 'FL10000'); %moves 10000 steps clockwise

fprintf(s2, '%s\r', 'VE1'); %velocity to 1 rev/sec

fprintf(s2, '%s\r', 'FL10000'); %moves 10000 steps clockwise

fprintf(s1, '%s\r', 'VE1'); %velocity set to 1 rev/sec

fprintf(s1, '%s\r', 'FL-10000'); %moves 10000 steps counter clockwise

fprintf(s2, '%s\r', 'VE1'); %velocity set to 1 rev/sec

fprintf(s2, '%s\r', 'FL-10000'); %moves 10000 steps counter clockwise

fclose(s1)

fclose(s2)

delete(s1)

delete(s2)

clear s1

clear s2