The command Union(cs1, cs2, R) returns a constructible set, the union of cs1 and cs2.

There might be redundancy in the output.

This command is part of the RegularChains[ConstructibleSetTools] package, so it can be used in the form Union(..) only after executing the command with(RegularChains[ConstructibleSetTools]).  However, it can always be accessed through the long form of the command by using RegularChains[ConstructibleSetTools][Union](..).