Mac Pro 2 x 3.2 GHz Quad-Core Intel Xenon 16 GB Ram using Logic 8
I can't play more than one instrument at a time, either in the stand alone
or when opened up in Logic. I know it must be a simple fix - but it eludes me.
You don't provide enough information. I infer that you try to play several sounds layered from a keyboard? To do that, you have to assign the same MIDI channel to the instruments that have to play together.