The LED light of machine B can be lit by controlling the buttons of machine A, and numbers can be sent to machine A by controlling the buttons of machine B, and displayed using a digital tube.

Circuit diagram:

Machine code:

#include
#define uchar unsigned char
#define uint unsigned int 

sbit LED1=P1^0;
sbit LED2=P1^3;
sbit K1=P1^7;
uchar Operation_No=0; //Operation code
//Digital tube code
uchar code DSY_CODE[]={0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,0x6f};

//Delay
void DelayMS(uint ms)
{
	flying i;
	while(ms--) 
	{
		for(i=0;i<120;i++);
	}
		
}
//Send characters to the serial port
void Putc_to_SerialPort(uchar c)
{
	SBUF=c; 
	while(TI==0); 
	TI=0; //TI hardware set to 1, software cleared to 0
}
//Main program
void main()
{
	LED1=LED2=1; 
	P0=0x00;
	SCON=0x50; //Serial port mode 1, allow receiving
	TMOD=0x20; //T1 working mode 2
	PCON=0x00; //Baud rate is not doubled
	TH1=0xfd; 
	TL1=0xfd;   			//9600
	IF=RI=0;       //
	TR1=1;
	IE=0x90; //Enable serial port interrupt
	while(1)
	{
		DelayMS(100);
		if(K1==0) //Select operation code 0, 1, 2, 3 when K1 is pressed
		{
		while(K1==0); 
			{
				Operation_No=(Operation_No+1)%4;
			}
			
		 
		switch(Operation_No) //Send A/B/C or stop sending according to the operation code
			{
				case 0:    Putc_to_SerialPort('X'); LED1=LED2=1;break;
				case 1:    Putc_to_SerialPort('A');LED1=~LED1;LED2=1;break;
				case 2:    Putc_to_SerialPort('B'); LED2=~LED2;LED1=1; break;
				case 3:    Putc_to_SerialPort('C'); LED1=~LED1;LED2=LED1; break;
			}
		}
	}
}
//Serial port receiving interrupt function of machine A
void Serial_INT() interrupt    4
{
	if(RI)
	{
		RI=0;
		if(SBUF>=0&&SBUF<=9) 
		{
			P0=DSY_CODE[SBUF];
		}
			
		else 
		{
			P0=0x00;
		}
			
	}
} 

12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182

Machine B code:

#include
#define uchar unsigned char
#define uint unsigned int 
sbit LED1=P1^0;
sbit LED2=P1^3; 
sbit K2=P1^7; 
fly NumX=-1;
//Delay
void DelayMS(uint ms)
{
	flying i;
	while(ms--) 
	{
		for(i=0;i<120;i++);
	}
		
}
//Main program
void main()
{
	LED1=LED2=1;
	SCON=0x50; //Serial port mode 1, allow receiving
	TMOD=0x20; //T1 working mode 2


	TH1=0xfd; //Baud rate 9600
	TL1=0xfd;
	PCON=0x00; //Baud rate is not doubled
	SEE=OF=0; 
	TR1=1; 
	IE=0x90; 
	while(1)
	{
		DelayMS(100);
		if(K2==0)
		{
			while(K2==0);
			NumX=++NumX; //Generate a number in the range of 0~10, where 10 means off
			SBUF=NumX; 
			if(NumX==10)
			{
				NumX=-1;
			}
			while(TI==0); 
			TI=0; //TI hardware set to 1, software cleared to 0
		}
	}
}
void Serial_INT() interrupt 4
{
	if(RI) //If received, LED will be activated
	{
		RI=0;
		switch(SBUF) //Complete different actions according to the different command characters received
		{
			case 'X': LED1=LED2=1;break; //all off
			case 'A':  LED1=0;LED2=1;break;    //LED1 亮 
			case 'B':  LED2=0;LED1=1;break;    //LED2 亮 
			case 'C': LED1=LED2=0; //all on
		}
	}
}123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263

Effect screenshots: