[f-nsp] Multi-module trunk group

[Andrea]

Hi, I'm trying to configure some trunks on my bigiron-rx-8
with IronWare Version 2.2.1gT143

For redundacy reason I'd like to put in trunk group two ports in
different module, but the command is always rejected, example

slot 2 - 24 sfp module with 1000sx sfp (RX-BI-24F)
slot 6 - 24 sfp module with 1000sx sfp (RX-BI-24F)

command:
telnet at bigiron(config)#trunk eth 2/7 to 6/1
error - one port range must be within one module

I read documentation and I've checked that both ports have
the same vlan configuration, no ip address, acl and so on,
from foundry website seems tha it cuuld be done
http://www.foundrynet.com/services/documentation/bigiron_rx_config/Trunking.html#wp78363
but no example are present

Am I misunderstanding something?

Thanks in advance
Andrew