I only have the one ZBT-1 at the moment, but like you I was also thinking about getting a second for Thread (though in the form of a HA Yellow). There’s this bit in the documentation for the Yellow about multiprotocol support and one of the things is that it’s better to have a second wireless chip in some cases which I take to mean that you can have multiple ZBT-1, one for Zigbee and one for Thread.
For large networks it is advisable to add a second wireless chip. This allows to run Zigbee and Thread each on a dedicated wireless chip and on separate channels.
I only have the one ZBT-1 at the moment, but like you I was also thinking about getting a second for Thread (though in the form of a HA Yellow). There’s this bit in the documentation for the Yellow about multiprotocol support and one of the things is that it’s better to have a second wireless chip in some cases which I take to mean that you can have multiple ZBT-1, one for Zigbee and one for Thread.
(source: https://yellow.home-assistant.io/guides/about-multiprotocol/)