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[penultimatedoomguy]

I am specifically talking about Open MPT.  I am not sure how to use another patch from the same plugin for another instrument.  I am using SQ8L so I can decide if I really want to make music on an Ensoniq ESQ-1.

[Saga Musix]

I'm not sure I understand your question correctly, but if you want to use an instrument designed in SQ8L to be used on an ESQ-1, you can sent it to the synths using File -> Send Program in the SQ8L GUI.
In case that your question was how to use two different patches in the same song, SQ8L is a monotimbral plugin so you will have to load up two instances of the plugin.

[penultimatedoomguy]

And how to add another instance? Also, you see, I am looking forward to making music on synthesizers.  I am saving up for a synthesizer, and I can't decode if I want a Korg Minilogue or an ESQ-1.  I know they're very different, but they're cheap too.

[Saga Musix]

You add another instance just the same way as you added the first instance. So either through the general tab by selecting another instance into an empty plugin slot, or by creating a new instrument and assigning it to an empty plugin slot, which will then bring up the plugin selection dialog.