Although these two nightstands hail from opposite ends of the style spectrum, they share a lot of parallels in form. Both are tall and leggy, with generous open space underneath. They're about the same width, and their visual weight is similar. Twin mirrors and lamps help to enhance the tables' similarities and downplay their differences.
Try painting mismatched stands in a single color to make them harmonize. This pair of nightstands isn't identical, but the consistent finish and shape give the impression that each is part of the same family.
In a similar way, the handsome wood on this console table and chest of drawers lets them marry well within the setting.
If your night tables share absolutely no common threads, use other elements of the decor to tie them together. Here, the lamps and window treatments help to make the nightstands look of a piece, even though they differ in finish, form and feel.
Of course, you could always take the opposite approach: Emphasize their differences. Using a single lamp exaggerates the asymmetrical feel of these two tables.
This duo reminds me of the old nursery rhyme about Jack Sprat and his wife. What makes them work well together? The shorter one is topped by a lamp that adds a little height, while a low-slung stack of books tamps down the taller one. The symmetrical artwork over each balances out the composition.
This space gets away with mismatched nightstands largely because the one on the far side of the bed feels so independent of the other (see the next photo for a closer look).