Sure you can. However here are some things to keep in mind Assuming yours is a stereo setup: - Make sure you maintain left right speaker pair connections correctly - Maintain speaker connection polarity - dont interchange the connection pair to each speaker.
Also remember the amplifier will now be driving 4 speakers and hence needs the required power rating for this. Some amps have A/B outputs that allow for connecting two pairs of speakers.
The amplifier doesn't know it's driving 4 speakers, but (maybe worse) it sees its load going up from 8 omhs to 4 ohms (assuming all speakers are 8 ohms). So make sure your amplifier/receiver can handle that load.
Also, since the speaker impedance (e.g. nominal 8 ohms) actually varies a bit with frequency, combining different speaker models might sound weird.
Anonymous: I'll add to what Peter wrote (which was excellent). You need to check the impedance of your speakers. By connecting them in parallel, you will halve the load seen by your receiver. That is what Peter was writing about. If you connect -- in parallel -- two 8ohm speakers you receiver will see a 4ohm load (this is known based on Ohms Law).