Rainwater harvesting: Downpipe used horizontally

[yazsan83]

I am installing a rain water harvesting system from two sides of double garage. Since the width of the garage is long enough, I need to connect two gutters with two down pipes installing them horizontally.
In the connecting section of these two downpipes, there's a water leak.

The downpipes are installed with a slight slope.
Would you suggest the best way to stop this leakage? Any specific parts (I doubt)? Or impossible to use downpipes horizontally?

 

[Alastairreid]

Rainwater connections are not watertight, you can try clear silicone in the join.

 

[yazsan83]

Many thanks for your prompt advise.
Since I noticed the leakage, I used this product extensively; inside and outside of the join.
http://www.tec7.ie/
Still a bit of leak.
Maybe I should have used clear sealant.....
Or I need to give more slope.