aggregate bandwidth of 2 server each with 2 nics

Hi,

I'm trying to increase the bandwidth between 2 hosts in the same
subnet/vlan (/24). Both server are equipped with 2 GbE NICs and located
in different buildings which are connected with 3 x GbE. Both
interfaces (eth2/3) have an ip address and if needed I can use third ip
for each host.

First I tried bonding with Cisco post channels on the switch. But this
is not working because of the src/dst hash that Cisco uses to decide
which interface in the port channel is used. I want to increase the
bandwidth between 2 server and for the same session (backup), thus the
src/dst mac/ip is always the same.

I followed different HowTOs to solve this problem on layer 3 (policy
routing, equal cost multipath routing). But I only get the max. speed
of one interface with netio, netpipe or any other network benchmark. I
know that I won't get the full transfer rate of both interfaces (packet
reordering...), but 20-30% more than a single interface should be possible.

http://lartc.org/howto/lartc.loadshare.html
http://lartc.org/manpages/ip.txt

I'm using debian etch with kernel 2.6.21 from backports.org, because
multipath routing is broken in 2.6.18.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=418344

Does anyone have a working example of bandwidth aggregation between 2
hosts in the same subnet for a singe tcp session?

Thanks, Ralf