For a reaction R₁ , $∆$G = x KJ/mole, for a reaction R_2, $∆$G = y KJ/mole. Reaction R₁ is  

non-spontaneous but along with R₂ it is spontaneous. This means that

A process is non spontaneous means $∆\mathrm{G}=+\mathrm{ve} ;$

Thus for $$\begin{gathered} {\mathrm{R}}_1, ∆\mathrm{G}=\mathrm{x}=+\mathrm{ve}; \\ \mathrm{But} \mathrm{for} {\mathrm{R}}_2 ∆\mathrm{G}=\mathrm{y}=-\mathrm{ve} \mathrm{because} \mathrm{it} \mathrm{is} \mathrm{spontaneous}; \\ {\mathrm{R}}_{1 }\mathrm{becomes} \mathrm{spontaneous} \mathrm{in} \mathrm{presence} \mathrm{of} {\mathrm{R}}_2, \\ \mathrm{this} \mathrm{is} \mathrm{possible} \mathrm{only} \mathrm{when} \mathrm{magnitude} \mathrm{of} \mathrm{y}>\mathrm{x} ; \end{gathered}$$

Such reactions are known as coupling reactions.