Current sources should never be combined in series. What would happen if we connect two current sources with different currents 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 the terms in the right-hand side are taken with $+$ sign if the corresponding current source $I_i$ points towards the same node as $I_{eff}$ and with $-$ sign if $I_i$ points to the other node. Since all the 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 $I_1$, replace its value with $$\begin{equation}I_{1,eff}=I_1-I_2+I_3\end{equation}$$ and remove current sources $I_2$ and $I_3$ from the circuit. Notice that $I_1$ and $I_3$ appear with positive sign in the above equation because both $I_1$ and $I_3$ point towards node $v_3$, just like $I_{1,eff}$. $I_2$ appears with negative sign because it points towards node $v_4$ while $I_{1,eff}$ points towards node $v_3$. If we kept current source $I_2$, we had to replace its value with $$\begin{equation}I_{2,eff}=-I_1+I_2-I_3\end{equation}$$ $I_1$ and $I_3$ appear with positive sign in the above equation because $I_2$ points towards node $v_4$, just like $I_{2,eff}$. $I_1$ and $I_3$ appear with negative sign because they point towards node $v_3$ while $I_{2,eff}$ points towards node $v_4$. Also, if we kept current source $I_3$, we had to replace its value with $$\begin{equation}I_{3,eff}=I_1-I_2+I_3\end{equation}$$