I figure it out.
Flags
32bit
Causes the {sys} constant to map to the 32-bit System directory when used in the Filename and WorkingDir parameters. This is the default behavior in a 32-bit mode install.
This flag cannot be combined with the shellexec flag.
64bit
Causes the {sys} constant to map to the 64-bit System directory when used in the Filename and WorkingDir parameters. This is the default behavior in a 64-bit mode install.
This flag can only be used when Setup is running on 64-bit Windows, otherwise an error will occur. On an installation supporting both 32- and 64-bit architectures, it is possible to avoid the error by adding a Check: IsWin64 parameter, which will cause the entry to be silently skipped when running on 32-bit Windows.
This flag cannot be combined with the shellexec flag.
Source: http://www.jrsoftware.org/ishelp/index.php?topic=isxfunc_iswin64
Based on this, i changed my script ir order to pinpoint correctly the "msiexec.exe" file for both architectures. Thank you guys!
[Run]
Filename: "{sys}\msiexec.exe"; Parameters: "/package ""{userdesktop}\Classic_Client_{#MyAppVersion}_64.msi"" /qn /norestart /passive"; Flags: 64bit skipifdoesntexist waituntilterminated; Check:IsWin64; StatusMsg: "A instalar Classic Client {#MyAppVersion} - 64bit";
Filename: "{sys}\msiexec.exe"; Parameters: "/package ""{userdesktop}\Classic_Client_{#MyAppVersion}_32.msi"" /qn /norestart /passive"; Flags: 32bit skipifdoesntexist waituntilterminated; StatusMsg: "A instalar Classic Client {#MyAppVersion} - 32bit";