Instead of cpl p2.1 use setb and clr to ensure that no other display is active...

Definitely use timer interrupt...

Simply an interrupt is something that tells you an event has happened. These events might be a falling edge or high level in some pins (int 0 and int 1), an overflow in some registers like (timer,counter) or a signal that data in serial buffer is ready. If you set your hardware and program to use these interrupts you can solve your problems dead easy. For example if you want to communicate with another device using serial port how will you detect data is ready? will you poll RI bit (like you do in TF0)? Instead, wouldn't it be good to consume you uC cycles to do the jobs like displaying values , scanning buttons and inputs and only when your interrupts occur to call necessary codes.