The Portuguese look like the Spanish, Portuguese sounds similar to Spanish. Therefore the Portuguese and the Spanish must be one and the same.
Portugal became a country in 1139, centuries before Spain. The Portuguese have different customs and traditions. Both countries languages have similarities due to the fact that they come from Latin, but so did French and Italian. All these languages have the same origin however they have developed to become very different from each other.
Naturally Portugal and Spain influence each other, but that is not something unique to them. Most countries that share a border have an effect on each other. Looking close to home, the United States influences Canada, yet that does not mean that the Canadians and Americans are the same.
If one goes to Portugal or Spain and tries to convince the people there that the two countries are one and the same, one would not find great support. Two independent countries are exactly that, independent.