In order to create multiple custom charts for the same pair (symbol), you need to attach the custom chart indicator to the symbol's M1 chart each time you want to add an additional custom chart.
IMPORTANT: Always use a different offline time frame for each chart and make sure that you are not using the standard MT4 time frames like (1,5,15,30,60, etc.).
An example setup for 2 median renko charts for EURUSD:
- chart 1 will have 7 pip bars ( I use 3 for the offline time frame )
- chart 2 will have 21 pip bars ( I use 4 for the offline time frame )
Your M1 chart will look like so: