We analyze the situation when the original graph is split at edges and vertices into two disconnected subgraphs. We show that there is a relationship between the generalized Euler characteristic Eo(|VDo|) of the original graph and the generalized Euler characteristics Ei(|VDi|), i = 1, 2, of two disconnected subgraphs, where |VDo| and |VDi|, i = 1, 2, are the numbers of vertices with the Dirichlet boundary conditions in the graphs. Theoretical predictions are verified experimentally using microwave networks which simulate quantum graphs.