nested mpirun commands

Hi,

   I have an mpi program that calls another mpi program (written by someone else) using a fortran system call

driver.f90

call MPI_INIT(..)

system('mpirun -np 2 ./mpi_prog.x')

call MPI_FINALIZE(...)

When I run it (e.g. mpirun -np 4 ./driver.x) it crashes inside mpi_prog.x (at a barrier). When I build it using mpich it works fine though. Any hints on what might be wrong (I realize nested mpirun's are completely beyond the mpi standard and highly dependent on implementation)

NOTE: when I do something like:

mpirun -hosts hostA,hostB -perhost 1 -np 2 mpirun -np 4 hostname

it works perfectly fine and returns the correct output.