You can either call the .MSI/EXE containing .MSI within the USER Interface or within a finish dialog.
1.) Add an Execute Program from Destination action. You will call msiexec.exe and pass the command line. If you wanted to call the .MSI silently, then you can pass the following command line: /i <full path to .MSI> /q
2.) You would use the Execute Program from Installation custom action to execute the .EXE containing the .MSI.
3.) The better option is to create your own .EXE wrapper and then call the .MSI's in the desired order using Wisescript.