Each chart can be used to analyse multiple symbols and/or different data intervals at the same time. It's not recommended to use Tick or Volume data intervals in Multi-Data charts, because not every data provider supports under-minute time resolution in supplied historcal data.
Data charts are numbered from 1 to Number of symbols on chart. Every analysis technique has Based On parameter that specifies chart data number used to calculate that analysis technique. Inside analysis technique Data reserved keyword can be used to access required symbol data.
open = Data.Open; // open of last bar of symbol chart #2
Using syntax described above you can access any data described in Data Available in Analysis Techniques topic for any symbol applied on the chart.