From your above configuration, since both has host*, it will forward to the first indexer.
You can have multiple instances of DEST_KEY=_TCP_ROUTING at least as per the documentation in http://docs.splunk.com/Documentation/Splunk/6.0/Forwarding/Routeandfilterdatad under the heading
Filter and route event data to target groups
From your original question, lets take forwarder1,forwarder2 be the two forwarders which send data to indexer1 and indexer2 respectively.
In this case, the following should work,
props.conf
[host::forwarder1]
TRANSFORMS-routing=indexer1
[host::forwarder2]
TRANSFORMS-routing=indexer2
transforms.conf
[indexer1]
REGEX=.
DEST_KEY=_TCP_ROUTING
FORMAT=indexer1_group
[indexer2]
REGEX=.
DEST_KEY=_TCP_ROUTING
FORMAT=indexer2_group
outputs.conf
[tcpout:indexer1_group]
server=indexer1:9997
[tcpout:indexer2_group]
server=indexer2:9997
If you have separate source or sourcetypes from each of these forwarders, then you can mention it in props instead of hostname
Eg:
[source::<sourcename>]
TRANSFORMS-routing=indexer1
hostname and sourcename can be replaced by a valid regex as well
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