Is following MAC schemes formed by MAC schemes secure?
In the first case, $\Pi_3$ is not secure. 

We can use $(m_1, m_2)$ and $(m_3, m_4)$ to query the adversary $A$.

$t_1||t_2 = A(m_1, m_2)$

$t_3||t_4 = A(m_3, m_4)$

The forged MAC would be $t_1||t_4: Verify(t_1||t_4, (m_1, m_4)) \rightarrow Accept$.

In the second case, $\Pi_3$ is not secure.

We can use $(m_1, m_2)$ and $(m_1, m_3)$ to query the adversary $A$.

$t = t_1 \oplus t_2 = A(m_1, m_2)$

$t' = t_1 \oplus t_3 = A(m_1, m_3)$

$t_f = t \oplus t' = t_2 \oplus t_3$

The forged MAC would be $t_f: Verify(t_f, (m_2, m_3)) \rightarrow Accept$.

Another way to break the scheme is to reverse the message pair. $Verify(t, (m_2, m_1)) \rightarrow Accept$.