integral basis for $\mathbb{Q}(\sqrt a, \sqrt b)$

Question

I have a question regarding this.

Let $ gcd(a,b) = 1$, $K_1 := \mathbb{Q}(\sqrt a)$, $K_2 := \mathbb{Q}(\sqrt b)$

$\{1, \theta\}$ is the integral basis of $\mathbf{O}_{K_1}$(Ring of integers of $K_1$) and

$\{1,\theta^{'}\} $ is the integral basis of $\mathbf{O}_{K_2}$ (Ring of integers of $K_2$)

futher more let

$K := \mathbb{Q}(\sqrt a, \sqrt b)$ and $\mathbf{O}_{K}$ is the ring of integers of $K$

I don't understand, why you can write every $\alpha \in \mathbf{O}_{K}$ as

$\alpha = 1 \cdot \beta_1 + \theta \cdot \beta_2$ where $\beta_i \in \mathbb{Q}[\theta^{'}]$

I am new to algebraic number theory so I don't have any clue how to tackle this problem.