I know how to define a number field in sage by an irreducible polynomial over $\mathbb{Q}$, for example
sage: K.<a> = NumberField(x^3 - 2)

But how do I define any number field like $\mathbb{Q}(\sqrt{d_1},\sqrt{d_2})$ in sage, where $d_1$ and $d_2$ are two distinct squarefree integers? So how do I find the defining minimal polynomial of the field extension $\mathbb{Q}(\sqrt{d_1},\sqrt{d_2})$ over $\mathbb{Q}$?