Warp fields don't stack, i.e., generating two separate warp one fields will not result in travel at warp two, or even warp 1.4. As you say, they behave like car engines - you might be able to get more acceleration with two engines, but you won't increase your top speed.
The best example of this is in “Divergence” when the warp fields of the Enterprise and the Columbia are merged together. There is some turbulence when the warp fields first meet, but the ships maintain a steady course and Tucker transfers from one to the other, though the merged field, without being catapulted forwards. The tether is lost due to field fluctuations, but it falls behind the two ships, not forwards.
Additionally, going back to the beginning of Enterprise, the first human warp five drive was a very big deal, only possible through the genius of Archer's father and only after a long and expensive research project. If it were even theoretically possible to stack a bunch of warp engines together, they'd certainly have already tried.
This doesn't rule out the possibility in theory of using a number of small warp fields to move a larger vessel, but I don't think that's what you mean by "stacking" in this context. The events of “Divergence” does strongly suggest that such a piecemeal field would be very unstable, as does the fact that no known vessels in the Star Trek universe have more than one warp core.