There's only so much you can do in a high-wear environment like that. Digging tools use milder steel and are heat-treated for durability rather than edge-holding. Regardless of method, you're going to be resharpening on a regular basis.

I would certainly stay with a single bevel. It's not elegant, but it's faster to do and will probably wear longer. The angle of the bevel is a trade-off between penetrating ability and how long the edge will last. 45 degrees is the simplest and probably the best you can do in that application.

I sharpen all my digging tools by hand, with a 10" mill bastard file. For digging tools there's little value (IMO) in doing anything fancier. Frequent touch-ups keep the sharpening job from getting too big. If I'm far afield, a coarse diamond file does the touch-ups.

With a bigger fleet to maintain, hand sharpening probably isn't practical. A bench grinder or angle grinder will build up heat with frightening speed, even using the lightest pressure and fast passes. That's why knife knuts use a 1" belt sander. You could try Norton's white aluminum oxide wheels for bench grinders, designed to run cool(er) for sharpening tempered steel http://www.nortonindustrial.com/BenchPedestal-WhiteAO.aspx .

But consider also the contrarian view: when using a bench/angle grinder (with extreme care), the heat you generate will create a thin edge that is harder than the core steel of the tool, and will last somewhat longer. You'll know you've done this if a file bounces right off it. But if there is visible burning/blueing, you're doing damage.

Your new hori-hori may be a special case. I'm not familiar with that steel. It's probably a single bevel right from the factory, and you should follow those lines. But please use a file or diamond. I hope you won't put it on the grinder.

(Long post, sorry 'bout that. Practically a Sunday sermon. )