When you use multiple viewports, one of them is the current viewport, which accepts cursor input and view commands.
When a viewport is current, the cursor is displayed as crosshairs rather than an arrow, and the viewport boundary is highlighted. You can change the current viewport at any time except when a View command is in progress.
To make a viewport the current viewport, you click inside it or press CTRL+R to cycle through the existing viewports.
To draw a line using two model space viewports, you start the line in the current viewport, make another viewport current by clicking within it, and then specify the endpoint of the line in the second viewport. In a large drawing, you can use this method to draw a line from a detail in one corner to a detail in a distant corner.