If you are just running 2 clients you likely would be just fine with multiple solutions running 2 clients side by side on a single PC.
If the 2nd account is a healer or an intermittent DPS class (like a hunter who can send a pet and auto-attack or a warlock who could send a pet and quickly DoT and then start Wanding) then simple mouseover solution could work. If you want/need more fast window switching at a software level that is macro-able then I would look at WoW Open Box for a free solution or a JMB trial for a paid solution.
Not to denigrate Hardware based option but for 3 clients it just seems overkill to me to go with a KM switch and what not for simultaneous inputs for just 1 additional character like most hardware boxers are doing. If I were to go hardware based with your hardware I would maybe just get a small numeric keypad for the surface pro 3 to put next to (or just above) my main keyboard (or mouse) and hit that for the 2nd character.
There are also some addons which you may want which can help regardless of which route you go which include EMA, MAMA, & Dynamic Boxer.
A summary of the options can be found here:
https://docs.google.com/spreadsheets...gid=1566844114
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