1) To avoid unexpected behaviour/results always use “volatile” for variables in context with interrupt routines:

volatile unsigned int a;

instead of:

int a; // there are no negative numbers, so use “unsigned int”

2) always add a counter to check how many times there was a timer overflow to get a correct result (even if in this case an overflow may not be expected)

3) for measuring a pulse width you may also use the capture mode of the CCP module of your PIC (if available)

It is already explained above.
https://electrosome.com/hc-sr04-ultrasonic-sensor-pic/#Timer1_Module

Yes you can add another one. Just read data from each sensors one by one instead of doing simultaneously. When you do it fast, it will appear as simultaneous for us.

48 is the ASCII value of character ‘0’. Basically here we are converting an integer to corresponding character.

It is already explained above. Please read from below section.
https://electrosome.com/hc-sr04-ultrasonic-sensor-pic/#Timer1_Module

Yes it does, however you will have to make some subtle changes. For example, instead of simply typing “RBIF”, you will need to type “INTCONbits.RBIF”. There are many place you will need to do this in the code, including doing the same thing with “TMR1ON” which you would frame as “T1CONbits.TMR1ON”. Once you do that you should be fine. If you paste your code I will be able to tell you exactly how to frame it. Otherwise simply search the word following PIC18 and the syntax should come up on some forum.

Will this program work with the PIC18F4620?