I guess it would be hard, yes.
You would have players moving independently from each other over the same field. What if their moves are incompatible? Say I move a large stack onto the same hex as you do, and together we violate the stacking limit. I block a bridge in my move that you are about to use. I make contact with the same enemy units as you. I melee them, so do you. I retreat from a melee, into a hex that, unknown to me, your units have occupied in your move. Etc. etc. How to resolve such impossibilities?
I used to play 1812 as online MP game a couple of years ago. Worked nicely, except when Glyn Hargreaves and I, teaming up against two Indian gentlemen, sent our units into the same melee in the same moment. 200 Britishers meleed I think 15 Injuns. When the smoke cleared, the Injuns had stood their ground, but the British had vanished from the earth. 200+ redcoats, gone to the last man. The engine just wasn't able to cope with a melee situation involving units from two player commands. (Remember that movement on the same side happens simultaneously.)
Lot's of trouble there. A real Pandora's box. [|)]
<center>Général de Division D.S. "Green Horse" Walter
Baron d'Empire, Duc des Pyramides
Commandant [url="http://home.arcor.de/dierk_Walter/NWC/3_VI_AdR_Home.htm"]3ème Division[/url], VIème Corps Bavarois, L'Armée du Rhin
Brigade de Tirailleurs de la Jeune Garde

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