Two conducting spheres having radii R_1 and R_2 are placed widely...

The capacitance of an isolated spherical conductor of radius $R$ is given by $C = 4\pi\varepsilon_0 R$.

For the two spheres of radii $R_1$ and $R_2$, their individual capacitances are $C_1 = 4\pi\varepsilon_0 R_1$ and $C_2 = 4\pi\varepsilon_0 R_2$.

When the two spheres are connected by a metal wire, they acquire the same potential. The second plate for each spherical capacitor is considered to be at infinity, which is at a common zero potential. This means the two capacitors are effectively connected in parallel.

The equivalent capacitance of the combination is the sum of their individual capacitances:

$C_{eq} = C_1 + C_2 = 4\pi\varepsilon_0 R_1 + 4\pi\varepsilon_0 R_2 = 4\pi\varepsilon_0(R_1 + R_2)$