How to merge a parked domain with the same content as an addon domain in a subdirectory?

My new website is an addon domain in my cPanel. My old website is being retired and is parked on the same cPanel. I'm wanting to get the old website content served by one area of the new website. In effect the old site becomes a part of the new website. The new website (the addon domain) is in a subfolder immediately below public_html. The www.oldsitename.com redirection works if in cPanel I redirect to a specific file on the new domain name, but I need to handle about 20 pages from the old site. So I tried to redirect the old website to the addon domain subfolder and then using a .HTACCESS file in that space, and 301 redirects, redirect the individual pages to their new destinations. But that didn't work. I've tried various ways of doing it. Looking at the error logs all the different attemps are looking at the public_html directory for the files, which it never finds. I assume I could populate the .HTACCESS file in the public_html directory to handle the redirects but it seems to get messy with the main domain* in there aswell and especially with error pages when files are not found. Note: The old site is being used simply to keep the domain name and harvest the link juice. *A third domain occupies the main domain space and is unrelated to these websites. QUESTION: What is the best way to merge the old site, which is now parked, into part of the new website which is the addon domain in the directory public_html/new_site ?