So long as the Universal Forwarder on the Linux server is configured in outputs.conf to send its data to the Windows server, then adding the TA for Linux to it will mean that the Linux server will send its data too. You'll need to enable inputs on the TA on the Linux server. To do this, go to the default folder in the Linux TA, and copy inputs.conf to the local folder (if local doesn't exist, then create it). In local/inputs.conf delete all stanzas except those you'd like to enable, and then change the entry for that stanza to enable it. Off the top of my head I can't remember if it's disabled = true or enabled=false, but it should be quite clear.
You should also install the TA for Linux on your Windows server, but leave all inputs disabled as they don't apply to Windows machines.
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