If $L_1$ and $L_2$ are non regular then $L_1 \cup\;L_2\; = L$ can be regular?

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I need to prove or disprove with contrast example:

If $L_1$ and $L_2$ are non regular then $L_1 \cup\;L_2\; = L$ can be regular

I have no idea how to begin, hints and spoilers are welcomed

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How about $L_1=\{\mathtt a^n\mid n\text{ is prime}\}$ and $L_2=\{\mathtt a^n\mid n\text{ is composite}\}$?