Hello Kavya,
the parameter aCombine controls how actual operation (e.g SwitchToDialog) is combined with preceding operation. It doesn't control how SwitchToDialog should perform its own transition between the old and the new dialog. In case of SwitchToDialog such transition is always performed simultaneously - it is the switch. The parameter aCombine serves thus to group multiple, independent dialog operations together so they run all animations simultaneously.
If you you want to dismiss dialog1 and then present dialog2, then SwitchToDialog is not intended for this purpose. Use instead DismissDialog() and PresentDialog() in sequence. For example:
DismissDialog( dialog1, ..., false );
PresentDialog( dialog2, ..., false );
Best regards
Paul Banach