How to make two buttons act as a third?

[sju4sju]

I've got two Sega Saturn pads, which I wanna make a piggyback box for the XBOX, XBOX 360 and PS2 etc..
But I'd like to make it so that if I press the left shoulder button + start, it could for example send a Select button signal to the xbox joypad, or an XBOX 360 guide button signal.
What do I need to do this? I'd need a little logic chip or something? What was it called, AND, OR, XOR?

[SavannahLion]

It would be an AND operation. But an IC with an AND gate might not behave the way you want. You might need a pair of XOR and single AND gate to get the behavior you're looking for.

I don't quite understand what you're doing though. Are you soldering together your own arcade CP using a Saturn circuit board or are you building something that receives and interprets the signals from the game controller?

[sju4sju]

Thanks SavannahLion. Sounds complicated 
I'm just going to connect saturn joypads to XBOX360/XBOX controllers.. with 15pin joyport cables.

[SavannahLion]

Thanks SavannahLion. Sounds complicated 
I'm just going to connect saturn joypads to XBOX360/XBOX controllers.. with 15pin joyport cables.

I think I understand. OK, so you're planning on gutting the controller. I like doing less destructive ways, but it's a far more complex project and one has to start somewhere, right. 

I don't have direct experience with Saturn controllers, but I understand the control chip is really a dual multiplexer, turning the parallel button presses into a serial data stream. Looking for PCB photos and schematics, I did locate someone who did what I'm about to describe and even marks soldering points.

Look for the small IC and remove it. Then wire away. I was thinking you should use XORs with an AND. That might not be right since I tend to think in terms of always needing a high signal. Looks like it would be a two XNOR gates and a OR gate. Good luck finding that combination. You can construct your own using several NAND gates, but that would eat a lot of IC's. You can try PBJ's suggestion, go with IC's, or even convince someone to assemble a small circuit board with the behavior you want.

Another option would be to buy a small switch, drill a hole somewhere in the controller and wire it up using that. I wouldn't do that though. I don't really like modifying controllers like that.