When we calculate capacitance of single sphere shell , we assume outer shell is earth and it has infinite radius.
By Gauss's law the electric field at a distance r in between the shell and earth is
E.4πr2=q1ϵ0⇒E=q14πϵ0r2
The potential difference between shell and earth is ∫V0dV=−∫r1∞Edr
V=−q14πϵ0∫r1∞drr2=q14πϵ0r1
Thus the capacitance of A1 is C1=q1V=4πϵ0r1
Similarly for sphere A2 the capacitance C2=4πϵ0r2
Here both are connected in parallel so the equivalent capacitance is
Ceq=C1+C2=4πϵ0(r1+r2)