  Two planar graphs G1 and G2 sharing some vertices and edges are
`simultaneously planar' if they have planar drawings such that a shared vertex
[edge] is represented by the same point [curve] in both drawings. It is an open
problem whether simultaneous planarity can be tested efficiently. We give a
linear-time algorithm to test simultaneous planarity when the two graphs share
a 2-connected subgraph. Our algorithm extends to the case of k planar graphs
where each vertex [edge] is either common to all graphs or belongs to exactly
one of them.
