Answer: Both hardenbergia and trumpet vine are vigorous growers so while they will look attractive intertwined while they're young, one will eventually overtake the other. If this doesn't matter to you, go ahead and plant them in close proximity. If you want to preserve both vines, plant them some distance apart and allow them to meet somewhere in them middle.
In your hot summer climate I would give the trumpet vine some protection from afternoon sunshine. Filtered sunlight to full shade in the afternoon will make it happy.
Best wishes with your beautiful vines!
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