The normal operating range for a vast majority of amps is in the range of 4 ohms to 16 ohms total impedance PER AMP CHANNEL.
Speakers in parallel, the normal connection, divide, meaning the result is the impedance of a single speaker divided by the number of speakers.
Consequently, the only way to get two pair of speakers on an amp is if both speakers are 8 ohms, which results in a total of 4 ohms (8/2 = 4).
But, let's look at the application a little more; a small shop with only 10 people in it, roughly the size of a McDonalds? I must ask, do you really need four speakers?
If you have four, you can have roughly a uniform sound level throughout the shop, with two speakers, it will be louder near the speakers and quieter away from the speakers.
So, the point is, it depends on what kind of shop it is. If this is just casual background listening while people shop for general merchandise, the perhaps two speakers mounted up high so you are not getting direct sound, would be adequate. If you need more uniform coverage, the four speaker mounted up high.
If this is a bar or cafe, consider that there are probably some people who want to hear the music and some people who don't, or at least some people who don't want the experience dominated by music. In this case, two speakers in the prime party area would work, and from there the sound from the two speakers would soften a little across the room, so people could still have conversations.
So, the nature of how you want the sound to be in the room matters. Uniform coverage, or hot spots near and quieter spots farther away. The nature of the shop also matters. The experience of someone shopping for music is very different from someone trying to have a quiet cup of tea.
If you are going to have one amp and two pair of speakers, the core rule is that those two speakers must be no less than 8 ohms each (8 ohms or higher)
Now, there are some consumer amps, that are capable of more easily sustaining load below 4 ohms. NAD amps comes to mind, though I don't think it is recommended that you run them for extended periods with low impedance loads. I've never had a NAD, so I don't know the details, but based on the power specs being rated at well below 4 ohms, it implies that these load are workable.
For the record, you can get very good deals on the NAD C315 with 40w/ channel (£160 - SuperFi) and the massively powerful NAD C355 with 80w/ch but only in Silver and only if you walk into the store (£299 - Richer Sounds).
So, while I've given you the basic information about speaker impedance, it would help if you gave us some more information about your application. They type of shop, etc...?