Questions & Answers about El catálogo y el cancionero son muy largos.
In Spanish, it is normal to repeat the article before each singular noun in a list:
el catálogo y el cancionero
This makes it clear that you are talking about two separate things.
Leaving out the second el would sound unnatural here.
A good basic rule is: if you have two separate singular nouns joined by y, you will often repeat the article.
Because the subject is plural.
The subject is:
el catálogo y el cancionero = the catalogue and the songbook
That means two things, so Spanish uses the plural form of ser:
- es = is
- son = are
So:
- El catálogo es muy largo. = The catalogue is very long.
- El catálogo y el cancionero son muy largos. = The catalogue and the songbook are very long.