If you need to develop 16bit- and 32bit-versions of the same program simultaneously, you just have to consider the following:
Delphi 1 DFM files can be read by Delphi 2 and 3, but not vice versa. Start your work in Delphi 1, then port to Delphi 2/3.
Any Delphi 2/3 enhancements can be "protected" with IFDEFs in the .PAS files, so the same .PAS file can be used by all Delphi versions:
{$IFDEF ver80}
this is Delphi 1 specific..
{$ENDIF}
{$IFDEF ver90}
this is Delphi 2 specific..
{$ENDIF}
{$IFDEF ver100}
this is Delphi 3 specific..
{$ENDIF}
{$IFDEF ver120}
this is Delphi 4 specific..
{$ENDIF}
{$IFDEF ver130}
this is Delphi 5 specific..
{$ENDIF}
{$IFDEF ver140}
this is Delphi 6 specific..
{$ENDIF}
{$IFDEF ver150}
this is Delphi 7 specific..
{$ENDIF}
{$IFDEF WIN32}
this is specific to Delphi 2 and 3..
{$ENDIF}
{$IFDEF ver93}
this is specific to C++ Builder..
{$ENDIF}
{$IFDEF ver110}
this is specific to C++ Builder 3..
{$ENDIF}
The only serious incompatibility are the .DCR-files. You will have to load them with the Delphi 2/3 Image-Editor, and save them again to convert a 16bit .DCR-file to a 32bit one.