I use 2 sound cards (M-Audio Delta 1010LT), that i have tested individually and that work fine by themselves, yet when i plug both of them in, the second one always gives me slight clicks here and there. (Doesn't matter which of the 2 cards is in the second spot).
Typically i use a dac~ object with 14 output channels. But i implemented it for testing with a simple 2-channel mode, and then selected a channel on the second card in the I/O Channel settings, and it still made those clicking noises. Selecting a channel on the first card produced no noise.
The CPU-usage is usually high, but i also reduced it down to something moderate for testing purposes, and it still made noises.
To clarify the noise: it's not terrible, it's rather once in a while there are unwanted noises.