Someone thought of it already - a brand cuts diamond "parts" just for this. You may want to look down this link
The trick works reasonably well on small stones (Jenifer got a ring like that - quite a stunner!), but the gap between the two will be rather obvious on a large piece. I would rather J/I1, really. If you manage to find a brilliant marq with a whiff of fluorescence and discrete inclusions, that would look pretty darn great, I think.
To gte rid of the gap and create the perfect illusion, one facet of the "parts" would need to be flat... so that it fits flush with the next. I do not think such 'components' made to add up to a marquize exist. However, a version of "emerald cut" is out there: