If you created a second YouTube channel and you are want to link it to your original adsense account, then you are in the right place. It's not a hard thing to do. It's just that some of the peices are kind of hidden on YouTube's site.
I am going to assume that you have already created a second YouTube channel and that you want to link it to your adsense account. First, log into the YouTube channel that you want to tie to your adsense account.
After that, you need to click on the down-arrow next to your avatar in the upper-right corner of your screen, and choose "Settings".
Then you have to click on "Monetization" on the left side of the screen.
Next, click "enable my account", if you haven't done that already.
Read through the terms, and check the three boxes provided if you agree with them. Then click "I accept".
Click the "Got It" button.
Now, click on "Settings" on the navigation bar.
Then, click on "Monetization" on the left side of the screen again.
Click on "How will I get paid?".
Then click the "associate an adsense account" link (Chances are that this is the part you couldn't find).
Click the next button at the bottom of the page.
Then click on "Yes, proceed to Google Account sign in" to login with an existing google adsense account.
Log in with the credentials of the Google Adsense account that you want to associate with your YouTube channel.
The URL of your YouTube channel should load automatically.
Read the program policies, and if you agree select the check box, then click the "accept association" button.
And there you go, your new YouTube channel is associated with your other Adsense account.