Someone once asked me whether we can up-line a single-hand designated line and use it on his double -hander. The simple answer is yes but it will not Spey cast very well due to limitation of the design. It will no doubt over head cast well as it was designed to do just that but when it comes to Spey casting, there are some salient features needed for the line to Spey cast well. I will break up the Spey line into various sections and briefly discuss them here.
1) Section A-B:
This is the back taper of a typical Skagit or Scandi head. The back taper typically should be as short as possible so that it quickly merges with the thin running line (or shooting line). Since the purpose of the Skagit or Scandi Spey is to shoot line for distance, it make sense not to have too long a back taper. Typical back taper can range from a few inches to a no more than three feet. There are however exception to the rule. For the purpose of distance mending when fishing big rivers, RIO for example has incorporated a 25ft back taper to their RIO Switch line. This improved mending capability of course comes at the expense of shooting distance.
2) Section B-C:
This is the meat of the Spey line where most of the mass should be concentrated to optimize loading of the rod. Once this section of line is aerialized by the unloading action of the rod, it will gain the necessary momentum (= mass x velocity) to pull the rest of the line (C-D-E) along for the ride. You can think of B-C as the drive train and C-D-E as being the passenger carriages on tow. In order to go the distance, this lead section of the Spey line (B-C) should be as hefty as possible so as to provide maximum momentum for the shoot.
3) Section C-D:
This section of the line serves to link the power section of the line (B-C) to the anchor (section D-E) when a D-loop is formed. This section does not need to be as massive as B-C but should still have sufficient mass to turn over the sink tip (D-E) + fly. If this section is too light, it will fail to turn over the sink tip. If it is too hefty, it will put too much load on section B-C and slow down the overall line speed, resulting in a shorter cast. Rule of thumb is that this section must be less hefty than section B-C but must be at least as hefty as section D-E.
4) Section D-E:
In the case of a Skagit line, this section is the sink tip (or a long versi-leader in the case of Scandi). The main function of this section is to provide anchor for the cast. All spey cast require a good secure anchor to form before power can be applied effectively to the forward cast. If the anchor runs or slip, power of the forward cast will be lost and the cast will not go well. Another important purpose of this section is to turn over the fly. If we are using a heavy wind resistant fly, we will need a heftier sink tip (higher grain/ft) to provide sufficient momentum for turn over, Otherwise if we are casting dry fly or small nymph, we can probably get away using lighter grain weight versi-leader.
Here are the profiles of some of the better casting spey line out there that embraces this general design philosophy.