A stack is basically an set of speakers with an amphead on top of it. When you own several small amps it is possible to create you own, small stack. All you need is 2, or more, small amps. The amps do not have to be of the same make or model, you can use anything you have at the moment.
You will need a Y-cable which will run from your guitar output to the input of both amps. Instead of using a cable you can also use a Boss LS-2 lineselector. This is a pedal which will help to connect your guitar to several amps, or pedals, or other devices.
The advantage of playing through several amps will give you a unique sound: The sound is more complex compared to using just one amp. You guitar signal travels through several speakers, hence you do have a higher output signal and the sound is more complex compared to using only one speaker.
It is possible to set up each amp for a different sound, this will even create a more complex sound.
Once you start using several, small amps you may become interested in using larger amps. The Ministack is only one step away from playing through a larger ampset.
Enjoy your experiments and hope to see you soon again for more updates.