Actually, the frequency is getting one sample per iteration of the loop so your sampling rate will be dependent on how fast the loop iterates and that is subject to the whims of the operating system. If you want them to sample at the same rate you need to set them to do that. You have the 4431 sampling at 6kHz and the 6341 doing single samples. I'm not familiar with these devices but you should be able to either synchronize them or use two separate channels on one device for your inputs.