In fact there are three solutions:~
First solution: Some monitors have a plate at the back to which the support is attached. Often these plates have 4 screws. You can have a plate made to replace this one attached to a stand with four feet that will stand astride the base of the bottom monitor, bringing the bottom edge of the top monitor to line up with the top edge of the bottom monitor. Tools required: Metalworking tools. Material recommended: Aluminium. (Aluminum for our Amurican cousins).
Second solution: Look in google under the heading "monitor stands". There you will find several specialist firms manufacturing and distributing stands with swivel arms etc., Tools required: Credit Card.
Third solution: Take three pieces of wood. One piece to be the same width of the bottom stand plus twice the thickness of the wood. The other two pieces are cut to the exact height of the bottom monitor less the thickness of the wood. Now create three sides of a box. Join the egdes with wood glue. Drill holes and insert strengthening pegs to prevent wobble. With a jigsaw, cut out front facing edge of box in a curve to accomodate back of bottom monitor and to allow movement. To reduce weight, cut out a series of holes say one and a half inches in diameter, evenly spaced over the whole surface. This will reduce the weight considerably. Sandpaper off and varnish if desired. This will make a very effective and sturdy stand.
Tools required: Woodworking tools. Material: 12 ply beech board, wood glue, dowelling pegs.
Costs: No 3 cheapest and fun if you are a capable handyman.
Costs: No 1 Slightly more expensive if you are a capable metalworker.
Costs: No2 Very much more expensive if you are neither of the above.
However, if you are capable of taking measurements and making a working drawing (plan view, side elevation, front elevation) you could get someone to make it for you. But doing these things yourself is
great fun, cheap, and very satisfying.
Good Luck.