The OpenLogger was designed to be paired with and Arduino Pro or Arduino Pro Mini and just log the serial output from one of those. You could theoretically use the OpenLogger itself to do the sensor reading and writing, however, the I2C pins that your sensor needs haven't been broken out on the OpenLogger.
This means if you really wanted to make this happen, you would have to hand solder those pins and break them out yourself. After that the process is pretty straightforward. SparkFun provides both the source code and directions on programming it.
If trying to break those out is a little too tricky, you can either pair it with one of those Arduino models and use it to read the data, or find another similar logger device that has more pins broken out.
Two answer the second part of your question, reading multiple sensors isn't a problem. Usually the way these sensors work is you send a request for data to them and they return the reading, so you would just request from the first sensor, request from the second sensor, write out to the file.
You can write to two different files no problem eiteher. I would not however, recommend writing to two different files. Switching from one file to another, back and forth introduces a little extra overhead which will slow down your loop. In rocketry, things happen very fast, and you'll probably want keep your loop running as fast as possible. I would recommend just writing to a single file and include data from both sensors probably something along the lines of time,accelX,accelY,accelZ,bar.