Grates don't support eachother (or at least they didn't last version).
An easy way to get an early farm without much work is to just dig out an area next to or under a murky pool. Once you have the room dug, you dig out the tile to connect them and let the water flow in. If you dug it large enough the water will spread out to 2s and 1s and evaporate after a bit, giving you a big muddy farm area. Then you can wall off the spot where you connected them to close off the murky pool and it'll fill back up eventually with rainfall.
You can even make a deep underground water reservoir that collects rain water using a murky pool. Just make your reservoir then dig up/down stairs straight up into the murky pool until you breach it with a down stair. The water will flow right down the shaft to your reservoir. When the rain comes it will gather on the murky pool tiles and trickle right down the shaft into your reservoir. And the tile still counts as underground, since you did it with stairs. The great thing is you can even fish in this reservoir, catching whatever was present in the murky pool (usually turtles). It's safer than a river too, since even if it DOES overfill (if it rained enough to overfill your reservoir and start filling the shaft up), the rain is finite and generates fairly slowly - slow enough that it will simply evaporate before flooding anything, unless the room is very small and closed off.