We consider the problem of deforming simultaneously a pair of given
structures. We show that such deformations are governed by an L-infinity
algebra, which we construct explicitly. Our machinery is based on Th. Voronov's
derived bracket construction.
We consider algebraic and geometric applications, including the deformations
of morphisms of various kinds of algebras, of coisotropic submanifolds in
Poisson manifolds, and of twisted Poisson structures.
Our main goal in this paper is to translate the diagram relating groups,
Lie algebras and Hopf algebras to the corresponding 2-objects, i.e. to
categorify it. This is done interpreting 2-objects as crossed modules and
showing the compatibility of the standard functors linking groups, Lie algebras
and Hopf algebras with the concept of a crossed module. One outcome is the
construction of an enveloping algebra of the string Lie algebra of Baez-Crans,
another is the clarification of the passage from crossed modules of Hopf
algebras to Hopf 2-algebras.