Follow the instructions found here to send multiple MIDI messages with these particular instructions:
Your first subcontrol (called "Message 1" in the FAQ above) must be a:
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Button of type Momentary
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MIDI Message type Bank Select
Your second subcontrol (called "Message 2" in the FAQ) must be a:
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Button of type Momentary
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MIDI Message type Program Change
With this, MIDI Designer will produce a bank-select and program-change in one button press of the supercontrol.