Yes, you can connect and use multiple accounts for the same carrier, simply repeat the same process used to add the first account for your second, third, fourth (and so on...) accounts.
This feature really helps if your business has multiple shipping locations-- each with their own carrier accounts. For example, you might have an 'East Coast' location and a 'West Coast' location. Connect each of their accounts to ShipStation and give each one a unique nickname so you know which account is being used while you're shipping.
Notice that you have the option to choose a specific shipping account in the "Other Shipping Options" section of the Order Details screen.
A "Shipping Account" doesn't need to be specified every time you ship. By default, ShipStation will use a carrier's Primary account.
If you need to adjust this dropdown menu for a lot of orders, 'Apply a Bulk Action' to make a lot of changes at once, or use an Automation Rule to adjust orders as they import from your selling channels.
Also, this feature is not available with all carriers. Currently, Royal Mail is not able to link multiple accounts per carrier within ShipStation.