When you cascade counters, it is for adding extra digits. One counter counts 0 - 9, two count from 0 to 99, three count from 0 to 999. However, with two counters, there are 20 outputs if you uses 4017s, but there are always two outputs turned on, one representing the tens, the other representing the units.
There is no very easy way of getting two 4017 ICs to count to 0 - 19 and have only one of the 20 outputs come on at a time.
One solution is to connect the LEDs in two banks. Each bank has 10 leds. The positives are connected to one of your 4017 ICs, so two LEDs to each connection.
(LEDs 1 and 11 to output 1, LEDs 2 and 12 to output 2 etc.)
The connect all the negatives of LEDs 1 - 10 together, and all the negatives of LEDs 11 - 20 together. These you connect to outputs 0 and 1 of the second 4017.
Connect the "reset" connection of the second 4017 to its own "2" output. Connect the "clock" input of the second 4017 to the "carry out" connection of the first.
What should happen is that each time the first counter goes from 9 back to 0, there is a pulse on the "carry out" connection. The second counter counts those, so it counts up from 0 to 1. When it gets to 2, the 2 output is connected to the reset, so it goes back to zero. So either the 0 output is high or the 1 output is high.
The LEDs are wired from one of the outputs of the first counter to either the 0 or 1 output of the second counter. The only one that will light is the one connected to the output of the first counter that is high, and is connected to the output of the second counter that is low. The LEDs will not light when reversed.
If you want to go beyond 20 LEDs, you need to invert the outputs of the second counter. There is always only one output of the 4017s that is high. My scheme relies on only one of the outputs that is connected being low. With 20 LEDs, there are only two outputs connected, so only one being low is the same as only one being high, but with more outputs used, they all need to be inverted so that only one is low at any one time.