Piano question

Hi guys, I am making a piano mid library where you can play each note of the piano with just a single line, but I saw a android application Perfect Piano where they can mix two buttons sound at a time, so my question is are they really mixing the sound in real time or they are producing a new sound by just cheking the value lets say we have B3 + A4, so they play something like B7 ?

This seems more like a question people who specialize in the art of music might be more qualified to answer. I don't even understand the question.

Each note is based on a frequency... so odds are, they are running a calculation that combines the two (or more) frequencies into a new frequency, then producing a tone at that frequency. Typically that's how they work.

-tg

Ok that was helpfull, thank you I mannaged to do it with some high mat, but it was verry hard to combine them, because there is many variables which have to calculate in runtime.
I want to use this thread to ask another question connected with touch screens and exactly at what time the screen refresh information about the fingers on the surface I mean If there is a timer or something which collect data at 100ms are they enough for the software to detect two or even three fingers for that interval ? :)

"I want to use this thread to ask another question connected with touch screens " --- Nope, nope, nope... general rule here is one problem per thread... reason is pretty simple... no one is going to look in a thread with a "Piano question" to answer a touchscreen question... I suggest a new thread in the appropriate area.

-tg