static xdata unsigned char tbuf1 [TBUF_SIZE];
static xdata unsigned char rbuf1 [RBUF_SIZE];

static xdata unsigned char t_in1 = 0;
static xdata unsigned char t_out1 = 0;
static xdata unsigned char t_disabled1 = 0;

static xdata unsigned char r_in1 = 0;
static xdata unsigned char r_out1 = 0;

static xdata unsigned char tbuf2 [TBUF_SIZE];
static xdata unsigned char rbuf2 [RBUF_SIZE];

static xdata unsigned char t_in2 = 0;
static xdata unsigned char t_out2 = 0;
static xdata unsigned char t_disabled2 = 0;

static xdata unsigned char r_in2 = 0;
static xdata unsigned char r_out2 = 0;


/*------------------------------------------------------------------------------
------------------------------------------------------------------------------*/
unsigned char com_rbuflen1 (void)
{
   return (r_in1 - r_out1);
}
/*------------------------------------------------------------------------------
------------------------------------------------------------------------------*/
unsigned char com_rbuflen2 (void)
{
   return (r_in2 - r_out2);
}


or maybe:
/*------------------------------------------------------------------------------
------------------------------------------------------------------------------*/
unsigned char com_rbuflen2 (unsigned char port)
{
    if(port == 0)
    {   
       return (r_in1 - r_out1);
    }
    else
    {
       return (r_in1 - r_out1);
    }
}