g_spawn_async_with_pipes and pipes

[mathieu creationgif com]

Hello, I need to reproduce a shell function using pipe for example:
mkisofs args | cdrecord args 

So I would like to use 2 g_spawn_async_with_pipes and then use the same file 
descriptor in for one and out for the other. 

launch mkisofs
g_spawn_async_with_pipes(NULL,cmd_mkisofs,NULL,FLAGS,NULL,NULL,&pid_mkisofs,
null,&mkiout,null,null); 

then launch cdrecord
g_spawn_async_with_pipes(NULL,cmd_cdrecord,NULL,FLAGS,NULL,NULL,&pid_cdrecor 
d,
&mkiout,&cdrecord_out,&cdrecord_err,null); 

But, it doesn't seem to work.
Does somebody has a solution for me ?? 

Thanks a lot,