You should be able to create another tcp group in outputs.conf and then add that group to your _TCP_ROUTING setting in your inputs.
It sounds like you already have a tcpout group created for your indexers. So just create another for the other splunk environment. The _TCP_ROUTING setting in inputs.conf accepts a comma-delimited list of those groups, so just add that new group to the list. At that point, the eventlog data should get routed to both groups/environments.
If the other environment needs it in a different index, then they should be able to update their props.conf to set the right index before indexing on their side.
outputs.conf
[tcpout:<target_group>]
server = [<ip>|<servername>]:<port>, [<ip>|<servername>]:<port>, ...
* Required if indexerDiscovery is not set.
* Takes a comma separated list of one or more systems to send data to over a
tcp socket.
* Typically used to specify receiving splunk systems, although it can be
used to send data to non-splunk systems (see sendCookedData setting).
* For each mentioned system, the following are required:
* IP or servername where one or system is listening.
* Port on which syslog server is listening.
inputs.conf
_TCP_ROUTING = <tcpout_group_name>,<tcpout_group_name>,<tcpout_group_name>, ...
* Comma-separated list of tcpout group names.
* Using this, you can selectively forward the data to specific indexer(s).
* Specify the tcpout group the forwarder should use when forwarding the data.
The tcpout group names are defined in outputs.conf with
[tcpout:<tcpout_group_name>].
* Defaults to groups specified in "defaultGroup" in [tcpout] stanza in
outputs.conf.
* To forward data to all tcpout group names that have been defined in
outputs.conf, set to '*' (asterisk).
* To forward data from the "_internal" index, _TCP_ROUTING must explicitly be
set to either "*" or a specific splunktcp target group.
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