Yes you can. When you have more than one plug in a household then activity from either plug will be taken as being good.
What that means is that if you have an event Is Mum Up 6am to 10am and two plugs, one connected to the kettle, one connected to the TV.
Then if either the kettle or TV are turned on between 6am to 10am then you will get a reassurance message. If neither are turned on then you will get an alert.
There may be a higher charge for multiple plugs as each has a SIM installed with monthly data charges
Alternatively you can have multiple plugs in multiple households, in this situations events apply to the household only so you will get separate alerts or messages from you parents and your in-laws for example.