Current sources should not be combined in series. What happens if two current sources with different currents are connected in series?

If two or more current sources — $I_1$, $I_2$, ..., $I_n$ — are connected in parallel, they can be replaced with a single current source with $$I_{eff}=\pm I_1 \pm I_2\pm...\pm I_n$$ where each term on the right-hand side uses a $+$ sign if the corresponding current source $I_i$ points toward the same node as $I_{eff}$, and a $-$ sign if it points to the opposite node. Since all current sources are connected in parallel, when we replace $I_i$ with $I_{eff}$ the other current sources are removed (i.e. replaced with open circuits).

For instance, consider the circuit in Fig. 1, in which current sources $I_1$, $I_2$, and $I_3$ are connected in parallel. In this circuit, we can keep one current source, say $I_1$, and set its value to $$\begin{equation}I_{1,eff}=I_1-I_2+I_3\end{equation}$$ then remove current sources $I_2$ and $I_3$ from the circuit. Notice that $I_1$ and $I_3$ appear with positive signs above because both $I_1$ and $I_3$ point toward node $v_3$, like $I_{1,eff}$. $I_2$ appears with a negative sign because it points toward node $v_4$ while $I_{1,eff}$ points toward node $v_3$. If we kept current source $I_2$, we would set its value to $$\begin{equation}I_{2,eff}=-I_1+I_2-I_3\end{equation}$$ Here $I_1$ and $I_3$ appear with positive signs because $I_2$ points toward node $v_4$, like $I_{2,eff}$. $I_1$ and $I_3$ appear with negative signs because they point toward node $v_3$ while $I_{2,eff}$ points toward node $v_4$. Similarly, if we kept current source $I_3$, we would set its value to $$\begin{equation}I_{3,eff}=I_1-I_2+I_3\end{equation}$$