I can forsee at least two options
Option 1 – Connect both X Steppers, identical wiring, to the Motor 1 driver. You are connecting the motors in parallel. This is frequently done by folks who run GRBL and Gshields(only 3 on board drivers), more typically with X, 2 Y motors and Z, but there is really no difference here. If, for some reason, you believe one axis requires more torque for control, (i.e. X needs more than Y), then the axis needing more torque should get the individual drivers as two drivers can deliver more torque to the axis. The drivers are rated >2A, good airflow (fan) helps a lot.
Option 2 – two external drivers. The pre driver outputs are just connected to the logic ports on the uC, no special buffering. I would probably buffer the step and dir lines, although the need depends a lot on the input characteristics of your chosen external drivers. External drivers such as the Pololu DRV8825 are just driver devices same as used on tinyG mounted on a supporting PWB, Or, you can opt for higher end enclosed units. Many of the enclosed units have opto-isolator inputs and may need a logic buffer and possibly a 3.3V to 5V logic converter. Choice depends a lot on your board wiring skills.
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