I can see that the bigger picture is more complex. RadChart exposes a property called ItemsSource, which you can use. Each series mapping also exposes an ItemsSource property. When you don't set this property, the series mapping gets its items source from the chart's ItemsSource property. So there, you can use a different collection for each series you define.
If you only have these missing values (nulls) at the start and at the end of your table - you can use the previously suggested approach - remove these empty points, create two different collections and pass them to the corresponding series mapping.
I suspect that this will not be the case. If your data is fractional (12, 23, 1, 2, null, null, null, 56, 65, null, null, 54, 45, 11 and so on..) then a more complex solution is needed.
1. One thing I can suggest is to break the incoming data into small collections with no empty items in them. From the example above - (12, 23, 1, 2), (56, 65), (54, 45, 11). And then for each collection you can create a new series. If you go this way you will also need to put the legend into a manual mode. I have attached a simple app to demonstrate this.
2. You can try the Zero mode, instead of Gap and see if you like the results.
3. Another suggestion I can make is not to use the Spline series and use Line series, where such an issue will not exist.
the Telerik team