Its much easier and simple to just use one interface and that would be your sound card. When you plug an effect using its USB or Fire-Wire connection, it acts like and audio interface and you would have to use WDM driver mode as ASIO only supports one interface.
If you have a really good sound card with nice pre-amps, then you could get a a sound that is warmer and fuller depending on the pre-amps in your card.
Its a no brainer!
Cj