connect 2 graph to describe connection between 2 social networks

hey, i want to simulate spread into 2 diffrent graph, who connected with few links between them.

how can i merge 2 graph variables in matlab?

lets say

G1 = graph(....)

G2 = graph(....)

now i want to connect them to new graph that will has all their nodes and links + to add some links between them.

thank for help