If the goal is to learn about i2c
I would think that it would be much more useful to use 1 master and multiple slaves.
Which is a much more normal situation and is already supported by
the Arduino libraries.
So add another slave to the bus, like a realtime clock chip.
They are cheap.
You can also look at a i2c eeprom.
There are some pre-built RTC modules that have both a clock and
an eeprom on it.
This one has a i2c RTC an i2c eeprom and a 1 wire temperature chip:http://emartee.com/product/42059/Arduino%20Tiny%20RTC%20DS1307%20Shield%20V2.0