If i'm right, to control 2 max7219 i only ave to connect Dout of the first max to Din of the other max.
Correct. The clock and latch/load lines go to both chips in parallel.
but if I only want to send an instruction to the second max how can i do it? I've searched on google and forums but I haven't understood how it works.
There are two approaches.
1. Always update both chips each time. Its probably a good idea to keep a copy of the data to be displayed, on all your digits, in an array in your sketch. Write a function to send the entire array out to both chips. Each max chip contains two 8-bit shift registers. You send the instruction to one and the data to the other, for example the instruction might be "display the following data on digit 3" followed by the digit to be displayed. Then, before you tell the chip to accept/latch/load that data, you send another 2 bytes. As you send the second 2 bytes, first two bytes get transferred to the second chip. When all 4 bytes have been send, you then tell both chips to accept/latch/load the data.
2. There is a "null" command that you can send in the same way as above, so that if you want to update the first chip without disturbing the second, or vice-versa, you can send a null instruction. With two chips wired in series like this, you must always send 4 bytes before you tell both chips to load the data.
Is correct the schematic?:
Hmmm... hard to be sure as many things are not labelled, and the resolution is not very good. Can you get your schematic editor to label the pins with their names instead of their pin numbers?
Things to point out:
1. You should add a 0.1uF cap and a 10uF cap close to the power pins of each max chip.
2. Aren't some of your digits common-anode? If so, you have them wired to the max chips incorrectly. Your diagram would be correct if they were all common-cathode. As I said earlier, the max chip was designed for common-cathode displays, so the pins labelled "seg" should be connected to anodes and the pins labelled "dig" should be connected to cathodes. When using common-anode displays, you must still connect anodes to "seg" pins and cathodes to "dig" pins, even though this feels "wrong".
3. Not sure what is going on in the top-left of the diagram?