Hi Juan
It is worth taking the time to treat the pages as separate. The search terms can change from country to country and may mean that you are optimizing for a term that is not searched for in the English speaking country's that you are targeting. Have a look and see what your competitors are doing, this should give you a good starting point.
As you say it would be important to ensure that you did the link building differently for each website. English language websites pointing to the English page and Spanish websites to the Spanish pages.
Are you able to host the pages in different locations? If you could it would be worth doing so, you will find that this has a positive affect on the SEO.
You can also change the country that you are trying to target within Webmaster tools. (Sorry to write this if you know that already)
Hope that helps