Why not use a structure containing a type value and a union of the 2
structures that can be passed:

typedef enum
{
firststructure = 1,
secondstructure = 2
} MyStructures;

typedef struct
{
..........
} structure1;
typedef struct
{
........
} structure2;

typedef union
{
structure1 first;
structure2 second;
} MyUnion;

typedef struct
{
MyStructures structur_type;
MyUnion TheStructure;
} TheArgument;

Obviously you would want to name everything appropriately.