No matter howthe contract isput together, the key requirements for it to be legally enforceableare that: (i) the identity of the parties is clear; (ii) it must be clear what each party haspromised the other todo, or refrain from doing(this is called consideration and commonly consists of a promise by one partyto pay money in return for a promise by the other part to provide goods and/or services), and that each party has accepted the other party's offer (leaving somethingimportant "to be agreed" can be a contract killer); and (iii) it must be clear that the parties intended to enter into a legallyenforceable relationship. As long as those features are present the nature of the actual language used and the appearance of the document or documentsshouldn't matter.

The advantage of a written contract that is in one document that issigned by each partyis that (if it's clearly expressed) there should be better evidence of what was agreedand, hopefully, less room for disputes, than otherwise.