Datalogger storage calculation
Isn't this just basic maths?

1 sample per second * 60 seconds * 60 minutes * 24 hours * 30 days * 3 months = 7,776,000 points.

Assuming a 16 bit ADC that is then 2 channels * 2 bytes * 7,776,000 = 31.104 MBytes of data.

Even allowing for basic checksums and data formatting any data logger with the ability to log to an SD card should do the job.