Consider the following: imagine my two slings are joined with a girth hitch
A 'Girth hitch' does not qualify as a secure end-to-end joining knot (obviously).
Not sure why you felt the need to use it as an example? Using a girth hitch would put you in the running for a 'Darwin award' (if no single point of failure was a crucial objective).
Of course, Agent Smith's solution may be more practical overall, especially to the average user. Still, there are some situations where one must glean every inch of extension without sacrificing redundancy.
Forming an end-to-end rope join by tying
with bights is indeed a practical solution.
As I stated, the drawback with this approach is the amount of rope consumed to form the join.
If you really wanted to
maximize the overall length of your rope 'sling' - you should untie all of the knots so there are
no 'slings' to begin with. Then join both lengths of cord with a Zeppelin bend, to make one long length.
Next, form a round sling by using another Zeppelin bend.
Pass the entire round sling around the anchor (eg tree or boulder) and then unite the 2 end terminations with a simple overhand knot of perhaps an F8. This isolates both legs of the sling.
Job done.
I might add that my approach as described solves the problem of uniting the 2 round slings - because now there are
no round slings to unite! The 'problem' has been engineered out of the system. I like Zeppelin bends because they consume minimal rope, are totally jam resistant and secure and stable. Obviously, you need to be proficient at tying a Zeppelin bend