“Ejemplar en un mismo tenor y efecto” (or, more commonly in Spain, “ejemplar a un solo efecto”) is an expression used to indicate that each copy of a contract is considered an original. In English this most often appears in contracts simply as “counterparts, each of which is considered an original”. So I don’t think a literal translation is warranted (“the same tenor and effect”) given that English language contracts generally express this idea as both counterparts being “considered an original.”

Examples:

“This Agreement may be executed in separate counterparts, and such counterparts when executed and delivered shall be an original.”
http://www.drukker.co.uk/publications/reference/counterparts...

“This lease may be executed in any number of counterparts, each of which is considered an original.”
http://unterm.un.org/DGAACS/unterm.nsf/8fa942046ff7601c85256...

“This Memorandum of Understanding may be executed in counterparts, each of which is considered an original but all of which together constitute one instrument.”
http://www.courts.ca.gov/partners/documents/MOU-Template.doc...

“This agreement may be executed in several counterparts, each of which shall be deemed to be original but all of which will collectively constitute one and the same instrument.”
http://clausehound.com/category/agreement-workshop/commercia...

The distinction between “part” and “counterpart” may be UK usage and especially with respect to leases, but it is my understanding that the distinction is not generally made in many modern commercial contracts in English and it is not a feature of civil law contracts. Here are some references in that regard:

“Formerly ‘part’ was used as the opposite of ‘counterpart’ in respect to covenants executed in duplicate, but now each copy is called a ‘counterpart’. (A Dictionary of American and English Law, 1883, p. 927, quoted in Black’s Law Dictionary, 8th ed.)

“Formerly each party to an indenture executed a separate deed; that part which was executed by the grantor was called the original, and the rest the counterparts. It is now usual for all the parties to execute every part, and this makes them all originals.” 2 Bl. Com 296
http://legal-dictionary.thefreedictionary.com/Counterparts