No, you don't need to use them in tandem, sure, they are more effective if used in tandem, but not required.
And neither of them guarantees anything. Markets change, and you might be sitting in a drying oasis, behind a massive moat, surrounded by a desert of profitability.
And both moats and deserts have the drawbacks: moats defend from attacks but make you less mobile and they can easily expire (or be bypassed) with technological changes, just as stone walls became irrelevant with gunpowder, and Vauban-style fortifications became irrelevant with mobile warfare and aviation.
Deserts restrict your movements, since you can't try to move to adjacent markets to increase your profits, making it harder to handle declines in your core market.
And neither of them guarantees anything. Markets change, and you might be sitting in a drying oasis, behind a massive moat, surrounded by a desert of profitability.
And both moats and deserts have the drawbacks: moats defend from attacks but make you less mobile and they can easily expire (or be bypassed) with technological changes, just as stone walls became irrelevant with gunpowder, and Vauban-style fortifications became irrelevant with mobile warfare and aviation.
Deserts restrict your movements, since you can't try to move to adjacent markets to increase your profits, making it harder to handle declines in your core market.