How do we achieve the following using replace function in notepad++

Transform

Source: This is source 1
Description : This is the description of source 1
Source: This is source 2
Description : This is a description of source 2

into this:

Description : This is a source 1: This is the description of source 1
Description : This is a source 2: This is the description of source 2

All the examples I have seen append line 2 to line 1 like this:

(^Source.*$)(rn)
1

I need to append line 1 to line 2, and line 3 to line 4 etc.