To make dividing the lot in the future a viable option, this looks like a good way to lay things out. The lines show a 50' x 50' plot with a long driveway. 

This actually forms a bigger, more useful garage than my original plan, and still leaves a respectable yard for both. I won't be legally splitting the lot for this project but someone probably will later, and there's the possibility of making the back lot 56 or 60 feet deep rather than 50.

If the strip on the south side is shared easement, it leaves the option of building a garage at the back of the main house's property, at a right angle to the easement driveway. That would add covered parking as well as more privacy for both homes. But for my purposes, we'll just be widening the front driveway to provide parking for the main house. We'll expand the patio against the back of the house too, and we can use that to shuffle vehicles around if needed.

UPDATE: It turns out Portland has raised the lower limit on lot size to a minimum of 3000 square feet, so I would have to give the back 60 feet to the rear lot if it gets divided. That actually would be fine and leaves more room to expand the new structure. It just makes the front house's backyard tiny, and adding a garage there for the main house would be a tight squeeze.

I think I'll stick with this layout. The lengthwise arrangement I came up with earlier is harder to build and wouldn't look as good, even if it does leave slightly more yard.