I must admit I am baffled by how you would plumb in 2 sand filters unless they are both side-mounts and you use a single multi-port valve for both, and then it would probably need to be a larger and more expensive one. Otherwise I don't see how you could do it with 2 top mounts because of the multi-port valves. I guess they could be parallel, but then EVERY function would need to be duplicated on each, especially back-washing and you'd need to shut one down, than the other.

Comparing Hayward top-mount sand filters (Pro-Series):
An 18" (150#) filter runs about $230.
One with 22" (200#) runs about $250.
And a 24" (300#) runs about $380.

So I fail to see how a second 18" filter is cheaper than replacing it with a 22" or 24" filter. Plus if the 18" can be sold, even for far less, it offsets the cost of the bigger filter.