Since,
$\mathit{R}=\mathit{P}\frac{\mathit{L}}{\mathit{A}}$
$\mathit{I}=\frac{\mathit{V}}{\mathit{R}}=\frac{\mathit{V}}{\frac{\mathit{\rho }\mathit{L}}{\mathit{A}}}=\frac{\mathit{V}\mathit{A}}{\mathit{\rho }\mathit{L}}$
$\mathit{I}=\mathit{n}\mathit{e}\mathit{A}{\mathit{V}}_{\mathit{d}}=\frac{\mathit{V}\mathit{A}}{\mathit{\rho }\mathit{L}}$
when another conductor is joint in series


Now $\frac{\mathit{V}\mathit{A}}{2\mathit{\rho }\mathit{L}}=\mathit{n}\mathit{e}\mathit{A}{\mathit{V}}_{{\mathit{d}}^{\prime }}$
or, $\frac{1}{2}\left(\frac{\mathit{V}\mathit{A}}{\mathit{\rho }\mathit{L}\mathit{a}}\right)=\mathit{n}\mathit{e}\mathit{A}{\mathit{V}}_{\mathit{d}}$
or, $\frac{1}{2}\times \mathit{n}\mathit{e}\mathit{A}{\mathit{V}}_{\mathit{d}}=\mathit{n}\mathit{e}\mathit{A}{\mathit{V}}_{{\mathit{d}}^{\prime }}$
or ${\mathit{V}}_{\mathit{d}}=2{\mathit{V}}_{\mathit{d}}^{\prime }$
Drift velocity in second case will be half of the initial drift velocity
Current density $=\mathit{J}=\mathit{n}\mathit{e}{\mathit{V}}_{\mathit{d}}$
Initially current density $=\mathit{J}=\mathit{n}\mathit{e}{\mathit{V}}_{\mathit{d}}$
When another conductor injured in series current density ${\mathit{J}}^{\prime }=\mathit{n}\mathit{e}{\mathit{V}}_{{\mathit{d}}^{\prime }}$
$\frac{\mathit{J}}{{\mathit{J}}^{\prime }}=\frac{{\mathit{V}}_{\mathit{d}}}{{\mathit{V}}_{\mathit{d}}{}^{\prime }}=\frac{2\mathit{V}{\mathit{d}}^{\prime }}{{\mathit{V}}_{\mathit{d}}{}^{\prime }}=2$
∴ $\mathit{J}=2{\mathit{J}}^{\prime }$
Current density in second case will be half of the initial current density.