1) To increase the canopy size I want to plant them close together. What would be the appropriate planting distance between the two trees?
Actually with a mature spread of 25 to 35 feet, you should plant them as far apart as you can and have the mature canopies still overlap by 5 feet (this will allow enough room for both trees to mature in an even nature avoiding one or both canopies becoming lopsided thus more prone to breakage).....you take the spread witdth 50 feet (both trees mature spread).......divide by 2 (=25ft), as you only want the spread between the two trees not the amount of spread on the outside of them, now you subtract the amount of overlap you want (25ft - 5ft=20ft) So about 20 feet apart is the minimum, then the maximum follows the same logic......at 35ft it would be about 30ft.
2) How far from my home should these be planted to most benefit from the shade they will provide? (cutting electric/AC costs)
You must keep in mind the root system and how it will effect your foundation/driveway/patio etc.......generally the rule for trees is "that which is above is also below". There are some trees like beeches and (some) willow trees that do not follow this as their root structure is much more focused closer to the surface. Also you must consider how much overhang you want over your driveway/porch etc., and especially over your roof. Remember that the heavier the branch the more damage were one to come down, and as a deciduous (sp?) tree there will be leaves that fall into roof crevases and eves-troughs. Some is inevitable, but you can reduce the amount that falls on sides of the house that are not next to the tree. Personally I would go with the same range of 20 to 30ft. Plus I would place one on the southern side of the home and one on the west if at all possible, kind of splitting the corner if you like.......the suns angle during the summer causes the bulk of the suns rays to shine from the southwest. This I am basing on the assumtion that you live in the mid to northern United States.
3) Does adequate watering prevent roots from growing at the surface?
This I don't know, but you can contact your dnr office/extension and they will be able to tell you or direct you to the right resource.