Keeping Dogs in an Unfenced Yard?

I am looking for some advice. At the moment I own a lovable black Lab. We have two parks in either direction not far from my home where we walk and she has a fair sized back yard to play in. We attach her to a chain only long enough for her to go outside and do what she has to do and comes back in.

I'm considering adopting her a companion in the future, but unsure as to how I can manage both outside. There are busy streets surrounding our house where it would be too easy to wander off without a chain, if I attach another chair to our doorstep it would be impossible for either not to become tangled amongst each other.

I'd hate to put them in an outdoor cage, that would be cutting off less space for them, any advice on how I can manage two dogs outside on some sort of chains? I would like to let them roam as freely as possible without causing any danger to their well being, a large fenced in area is a possibility, but would would be somewhat expensive I imagine, but still a possibility.

By Jo