Do you need one plan or several? At one extreme, you can use your mobile phone to deliver internet connectivity to your tablet, notebook and other devices. At the other, you can arrange a separate connection for each. And there are variations in between. So how do you decide what to do?
These days it is unlikely that your carrier will prevent you tethering another device to your mobile phone, but it's always wise to check the terms and conditions. (The term 'tethering' originally referred to a wired connection between the devices, but is now also used for the situation where a phone becomes a Wi-Fi hot spot to share mobile data. Bluetooth can also be used, but Wi-Fi usually provides the sweet spot for convenience and speed.)
The main advantage of using a phone this way is that there's just one account to worry about.
The downsides are that the phone's battery will run down faster unless you already leave Wi-Fi active all the time, and you may experience 'bill shock' if you use more data than your plan includes, because some plans automatically top-up your allowance and charge accordingly - $10 per gigabyte seems the going rate. So you'd want to make sure your rarely-used notebook doesn't decide it needs to download a few gigabytes of software updates. If you're a consistently heavy user, you'll usually get better value by moving to a more expensive plan that includes more data, or by buying a data 'top up' in advance.
Completely independent connectivity is certainly possible. You can opt for the version of a tablet that includes 3G or 4G connectivity, and add a SIM. A small number of notebooks have that feature, but a USB wireless modem (aka 'dongle') is often cheaper and has more flexibility as you'll also be able to plug it into your next notebook.
If you select the same carrier, you may be able to pool the data among devices. One example is Optus's 'data pool'.The benefit is that if you use more data than usual on one device you can 'borrow' some from another without incurring extra charges. Another option can be to pay a relatively small monthly fee for an additional SIM that takes the data from your phone's plan (eg, Vodafone's $5 Red Business Grow MBB plan).
Alternatively, using different carriers for each device may increase your chance of remaining connected at all times. There are still some coverage black spots, and widespread outages have occurred recently (cough, Telstra, cough). You'll need to decide whether the value of that redundancy exceeds any additional cost - not just the difference in tariffs but also the administrative load of dealing with additional suppliers.
On the subject of redundancy, we know some small business operators that swear by long-expiry prepaid mobile broadband (eg, Telstra's $50 'annual pass' with 5GB of data) for this purpose. The trouble with prepaid is that you may run out of data at the most inconvenient time, so either take care to keep it topped up or make sure you have a way of loading a recharge that doesn't rely on being able to use the device.
Another approach that several of our small business contacts use is to reserve their mobile plan's data for the phone, and employ a portable Wi-Fi 3G or 4G modem to serve other devices. This provides very convenient connectivity for multiple devices, and where necessary it's easily shared with colleagues. All the major carriers offer such devices, you'll find Telstra's selection here. There are even versions designed specifically for in-car use, such as the Huawei CarFi.
Because the Wi-Fi modem is a separate device, it still works with your notebook or tablet even if your phone has run flat, and you only need one more SIM.
Again, you might choose to use the same carrier or a different one for the reasons outlined above. And depending on your pattern of use, prepaid may make more sense than going on a plan.
Everyone's situation is different, but here's our general advice.
If you can usually make do with Wi-Fi in offices, airports, hotels, etc for connecting your tablet or notebook, tethering to your phone when you just have to use mobile broadband is probably the simplest and cheapest approach.
If you regularly use multiple devices, a Wi-Fi modem is convenient and flexible. It can also serve as backup connectivity for your business premises if the fixed broadband service fails.