I would not recommend paralleling two drivers.
If you look inside the driver devices, you will see that current is dynamically managed. When a step pulse (or microstep pulse) is called for by the input, the driver output turns on, applying Vmot to the windings and current is monitored by a comparator circuit, comparing actual current flow in the winding to the settings pot on tinyG.
Most folks interfacing these sorts of high power loads use external drivers connected to tinyG.
If your motors really need that sort of current, you likely need a better thermal management solution than provided on tinyG.
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