We can interpret the question in two ways
Conider $Q_1,Q_2$ as current output
$D_1,D_2$ current input
$Q_1^{+},Q_2^{+}$ as next output
$1MHz=1000kHz$
Case 1:
considering $D_2=\overline{Q_1}$, $D_1=Q_1$
| $Q_1$ | $Q_2$ | $Q_1^+$ | $Q_2^+$ | $D_1$ | $D_2$ |
|---|
| 1 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 0 | 1 | 0 |
| 1 | 0 | 0 | 0 | 0 | 0 |
From the table we can see that the $Q_2$ complete 1 clock on 4 input clock or the frequency is $\frac{1000}{4}=250kHz$
(Repeat itself in 4 clock)
Case 2:
considering $D_2=\overline{\overline{Q_1}}=Q_1$, $D_1=Q1$
| $Q_1$ | $Q_2$ | $Q_1^+$ | $Q_2^+$ | $D_1$ | $D_2$ |
|---|
| 1 | 0 | 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 0 | 1 | 0 |
| 1 | 0 | 0 | 1 | 0 | 1 |
In this case the output $Q_2$ changes every 2 clock hence frequency is $\frac{1000}{2}=500kHz$