Strange problem about register window class

I want to register two window class in winmain.But when registering
the second window class,VC7.0 reports an error.The error code is 87 -
parameter error.But the parameters of both WINCLASSEX are the same
except the wndproc and wndclass name.

When compiling with bcc55,it's OK.When compiling with cygwin-g++-4.1.2
it's the same as VC7.0.

If assign wc.hInstance with
reinterpret_cast<HINSTANCE>(&hInstance) ,it's OK.Why?What does
&hInstance mean?

Below is the code:

// cl register.cpp -link gdi32.lib user32.lib
// g++ -o register.exe register.cpp -mwindows

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

LRESULT CALLBACK WndProc( HWND, UINT, WPARAM, LPARAM ) ;
LRESULT CALLBACK ProcSplitter(HWND hwnd, UINT Msg, WPARAM wParam,
LPARAM lParam);

void outError( UINT num )
{
    TCHAR buf[10] ;
    _stprintf( buf, TEXT("%u"), num ) ;
    MessageBox( NULL, buf, NULL, 0 ) ;
}

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE,
            PSTR, int )
{
    WNDCLASSEX wc ;
    WNDCLASSEX wndclass;

    wndclass.cbSize = sizeof(wndclass);
    wndclass.style = CS_GLOBALCLASS ;
    wndclass.lpfnWndProc = WndProc ;
    wndclass.cbClsExtra = 0 ;
    wndclass.cbWndExtra = 0 ;
    wndclass.hInstance = hInstance ;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION) ;
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW) ;
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH) ;
    wndclass.lpszMenuName = NULL ;
    wndclass.hIconSm = LoadIcon(NULL, (LPCTSTR)IDI_APPLICATION) ;
    wndclass.lpszClassName = "First" ;

    if( ! RegisterClassEx( &wndclass ) )
    {
        MessageBox( NULL, TEXT( "Can not register window class First" ),
                    "Error", MB_ICONERROR ) ;
        outError( GetLastError() ) ;
        return 0 ;
    }

    wc.cbSize = sizeof(wc);
    wc.style = CS_GLOBALCLASS ;
    wc.lpfnWndProc = ProcSplitter ;
    wc.cbClsExtra = 0 ;
    wc.cbWndExtra = 0 ;
    wc.hInstance = hInstance ;
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION) ;
    wc.hCursor = LoadCursor(NULL, IDC_ARROW) ;
    wc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH) ;
    wc.lpszMenuName = NULL ;
    wndclass.hIconSm = LoadIcon(NULL, (LPCTSTR)IDI_APPLICATION) ;
    wc.lpszClassName = "Second" ;

    if( ! RegisterClassEx( &wc ) )
    {
        outError( GetLastError() ) ;
        return 0 ;
    }

    return 0 ;
}

LRESULT CALLBACK WndProc( HWND hwnd, UINT message, WPARAM wParam,
LPARAM lParam )
{
    switch( message )
    {
    case WM_DESTROY :
        PostQuitMessage( 0 ) ;
        return 0 ;
    }
    return DefWindowProc( hwnd, message, wParam, lParam) ;
}

LRESULT CALLBACK ProcSplitter(HWND hwnd, UINT Msg, WPARAM wParam,
LPARAM lParam){
  switch (Msg)
  {
  default:
    return DefWindowProc(hwnd, Msg, wParam, lParam);
  }
}