Yes, but as both the Desktop VPN and Browser Extensions use the same secure encryption standards, using both at the same time doesn't offer significantly increased protection above simply using the Desktop VPN.
If you are connected to both the Desktop VPN and the Browser Extension, your connection will be routed twice, through each of the selected countries, meaning you might experience decreased browsing speed. Because of this, we recommend that you don’t use both at the same time, but only use the Desktop VPN for the best protection and browsing speeds.
However, when used at the same time, the Browser Extension will be active on top of the Desktop VPN. This means that your public location while browsing will be the one of the Browser Extension.