You could create a structure to store your float/double values and pass the address of that structure in the lParam value. If you are Posting the message rather than Sending it, you will need to get the recipient to free the memory occupied by the structure.
#define MYMESSAGECODE (WM_APP + 123 )
typedef struct
{
float f;
double d;
} MyDataStruct;
MyDataStruct data;
data.f = 1.0;
data.d = 2.0;
pWpfWnd->SendMessage( MYMESSAGECODE, 0, (LPARAM) &data );