It may be different on some devices but the general principle is you feed the first device in the chain, it's output feeds the next in the chain and so on. If you have two 8-bit devices chained together and you want to send data to the second one, you have to clock 16 bits out. The first 8 clocks fill the first device and the next 8 clocks move them out of that device and into the second one. You rely on the SPI devices not latching their shift register contents until the select pin tells it to, so you first align the data along the chain with the destination device then raise/lower it's select pin to tell it the data is there to be read. You obviously need a select line to each device to do this. The alternative is to parallel all the clock and data pins along the chain so they all see the same data but the individual select lines ensure only one device accepts it. So answering your question, in a serial chain it is possible to align the data for several devices at the same time and select them all simultaneously but it's rare to do that, in a more normal situation, regardless of which connection method you use, only one device at a time would be selected.
Brian.