If you have a Mac, it's OS can set up aggrigate (multiple) devices.
On Windows, ASIO is ONE device driver at one time.
IF the devices are from the same manufacturer and they support multiple devices on the chain it can be done.
(for example, Motu lets you chain up to 4 of their interfaces in one instance of their driver)
Otherwise, you will probably have problems or it won't work.
Another thing to consider is that unless you can clock the devices together, the individual
crystal clocks in each device beat at slightly different rates, so they will drift over time,
causing mis-matched tracks.
Best bet: Buy a bigger device with more i/o or one that supports multiple devices from
the same manufacturer and stop trying to figure out a way to get a free lunch....