Fig.: it's better to spread out the risks to avoid the possibility of losing everything if you rely exclusively on one person, one project, one plan of action or one idea

Do you really mean that your entire business is relying on just two customers? That's not enough! If I were you, I wouldn't put all my eggs in one basket. It's just too risky

Davvero tutta la tua attività dipende solo da due clienti? Sono troppo pochi! Fossi in te non punterei tutto su una carta sola, è troppo rischioso

Modern Portfolio Theory (MPT), developed by Harry Markowitz, is the technical finance theory equivalent to "Don't put all your eggs in one basket."

La Moderna Teoria di Portafoglio (MPT), sviluppata da Harry Markowitz, è la teoria della finanza tecnica equivalente a "Non mettere tutte le uova in un solo paniere".

 I was about to invest all my money in stocks, but my financial adviser warned me: “Don't put all your eggs in one basket”

Stavo per investire tutto il mio denaro in azioni, ma il mio consulente finanziario mi ha messo in guardia, dicendomi di non puntare tutto su una carta sola

This idiom comes from an old proverb, most likely Spanish or Italian, and is first found in print in “Don Quixote” (Part I, 1605) by Miguel de Cervantes:
“It is the part of a wise man to keep himself today for tomorrow, and not venture all his eggs in one basket.”
It is also found in “A Common Place of Italian Proverbs and Proverbial Phrases” by Giovanni Torriano, published in London in 1666.