I'm going to have to disagree on the statement that ASIO v2 allows for multiple devices. as glen said above, ASIO4ALL allows for multiple devices, but not the ASIO standard.
*ASIO4ALL is not a true ASIO protocol - it actually uses a soundcard device's WDM drivers and wraps them into an ASIO wrapper causing the host software to be able to use the ASIO internal calls, but the native uses on the computer are actually WDM. WDM DOES allow for multiple devices.
*ASIO does not limit the number of devices under certain circumstances. If a soundcard manufacturer creates hardware and software which are capable of running "as one device" under their ASIO compatible software drivers, then you can use more than one. you cannot, however, mix and match soundcards under ASIO drivers (such as the 44VSL and the UR22).
for example: MOTU creates ASIO drivers and hardware capable of being used together. one thing that needs to be done when doing this is one of them needs to be a slave and the other a master. several things play into this but the most important is the clock. one of them has to use the clock signal from the other device, so there has to be a connection between the two using the same clock signal. this could be through a PCIe, USB or Firewire bus or through an external cable connecting the two. each card could use its own clock independently as a stand alone unit under ASIO, but if using both together, one of them has to relinquish it's clock signal and use the clock from the other device.
If, however, you connect a MOTU soundcard and a Presonus soundcard and try to select ASIO in Sonar (or any other host), you will be forced to select either the MOTU ASIO driver or the Presonus ASIO driver - not both and both soundcards will not operate under Sonar (or any other DAW) using ASIO.
you can select WDM and use both soundcards under this scenario. However, as I mentioned above regarding the clocks when using ASIO - it is not possible to use the same hardware clock system in two cards which are not designed to be used as "one card." so if you select WDM drivers for a MOTU and a Presonus, you will still have to select ONE of them to be the master clock - but since they are not hardware physically using the same clock - they will drift away from each other when recording. small recordings might not ever matter. but a 5 minute song might very well show you that the clocks are not sync'd when using an input from each and an output set from one of them. longer recordings than that will absolutely show drift between the input channels.
ASIO standard is a consortium and held by Steinberg. it is a paper and a standard to which manufacturers design their software (and can design their hardware in conjunction with). the current version is 2.3. Software can share ASIO drivers if the software is designed to do so. Hardware (soundcards) can only share with other hardware if the hardware AND the software ASIO drivers from that manufacturer can support it. different manufacturers of soundcards will NOT support multiple ASIO devices.