I think the thread replies so far are missing the point of the issue...
Or maybe I am. But I think and @warawara correct me if I am wrong, that you want to have a different button then the select act as the hotkey button.
That or you do NOT want to select button to add coins.
I have done both in my builds. I like the coin acceptor for both the retro feel and to help limit play with myself and my kid.
Not to say it is the best solution but what I have done;
I wired the coin acceptor as a button and in the retropie I set it to "Left trigger" (as I don't use that in any of my games)
I then globally configure the emulators I use to use the left trigger instead of select for coin. It addresses 99% of the coin arcade games and there are a few custom deals that you have to work with. Player specific coin slot games are an example such as Simpsons where each player has a coin slot and it determines the character you play. For that I added a toggle switch that lets me "shift" the coin acceptor to the player I want to activate. But like I said, in most cases I'm fine leaving it on player 1.
Select in this method is just select and a hot key. No coins are added when pressed. This is basically the same solution as @DougA wrote with a few minor changes in what button the coin acceptor is assigned to.
For the other situation (IE independent hot key);
I had a small momentary puss button that I added to a build as a button for player 1. So player 1 had 9 buttons; 6 action, start select and this extra button. In the emulation station input configuration I set the extra button as the hot key. I only pressed that extra button when I got to the last entry of setting a hotkey. So instead of pressing select and start to exit you would press what I labeled the FN button and start. The same applies for all the other functions at that point as well. FN + B = reset, FN + X = menu and so on.
If we are way off base in what we are explaining, you will need to re-explain what you want to happen in your set up.
Ok, after some investigation, here are some afterthoughts. If I go in and remap the controller in Emulation Station, and switch my Start and Select buttons, it seems to be fine (other then not being mapped to the button I want them to be on the cabinet). With this set up, if I go into, say the NES emulator, the Start and Select work fine in the game and pressing both together allows me to exit the emulator. Now if I switch them back, Start works fine, Select immediately exits the emulator. If I map them to 2 other random buttons in my 6 button layout, everything works fine in the emulator, but pushing them together will not exit the emulator.
One thing that I did notice, is looking at the layout instructions that came with the Xin-Mo, it shows for player 1 that the Start button should be #9, and the Select should be #10. When I map the controller in ES, the Start button shows up as 9, but the Select shows up as 8. So, it's possible I have the Select button wired to the wrong input on the Xin-Mo board. This maybe a stupid question, but would it make that much of a difference? I mean, I'm telling it to map a control to a certain button.
I'm going to pull the control apart tonight and double check my wiring.