[LARTC] Multicst routing problem

[Zealot]

I want to send a multicast packet to multiple gre tunnel, but the packet 
only went to where the routing table configured to.

ex.
when use
    ip route add 224.0.0.0/4 via gre0
then the packet go to gre0

when use
    ip route add 224.0.0.0/4 via gre1
then the packet go to gre1

when use
    ip route add 224.0.0.0/4 nexthop via gre1 nexthop via gre0
then the packet go to either gre or gre1 randomly, but not both ( is it 
for load balance ? )

how to configure the routing table let the packet go to both of gre 
tunnels ?