I want to use more than one HiFiBerry Dac+ on a single Raspberry PI. It's possbile? to connect more than one audio card?
The Raspberry Pi has one I2S bus on its GPIO breakout header. I2S buses are the communications lines which connect the Pi's SoC to the sound card.
As there is only one I2S bus, it is only possible to connect one sound card to the Pi.
Another option for more then one sound card is to output to a I2S sound card and over the HDMI connector at the same time. This technique is addressed on the Audio Injector email list here.
If you are after more then two channels of output and input, multichannel cards are starting to appear for the GPIO header of the Pi. There is a short list of sound cards here.
Strictly speaking, if one card is a DAC only, then it may be possible to connect an ADC only card to the Pi on the same I2S bus, however only one of these cards could be configured to be the master of the bus. This is a detailed discussion which requires more background knowledge.
From HiFiBerry's support forum:
you can't use more than a single DAC+ on a Raspberry Pi. This is a technical limitation of the Raspberry Pi sound interface.