Thanks for all the suggestions!  Let's see if I am understanding them correctly:
 
Currently the reverb bus is being used by other tracks as well.  If I want to keep doing that, then it sounds like I will need to continue using 2 sends.  Promidi said that there could be a way to group the controls so that as I raise one send level, the other send level is automatically lowered.  This sounds like a neat trick, but I don't know how long it takes to set something like that up.  It's definitely something for me to try out when I have time.
 
For this particular song, it's a moot point, because I realized, after Craig pointed it out, that the effect on the reverb bus isn't 100% wet.  So, I can never get a "true" wet/dry range from 0% wet to 100% wet unless I create a second reverb effect which is 100% wet.
 
So, I have a couple options of where to create the second reverb effect: I can either create a new reverb bus or add the effect to the drum track's effects bin (in which case this would be called an Insert Effect, correct?).
 
If I create a second reverb bus (with a 100% wet reverb), to be used only by the drums, then I could set the drum track's output to the second reverb bus, and then automate the new reverb effect's wet/dry slider (assuming there is one).  In order for that to work, I would need to create a MIDI track for storing the reverb's automation, correct?  Or can we assign automation envelopes directly to buses too?  (I'm a bit fuzzy on effect automation). 
 
Of course, a simpler way would be to add the new reverb effect to the drum track itself.  I could then automate the effect's wet/dry amount directly within the drum track.
 
Does that sound correct?

1. If you will only be using the reverb bus for the drums you might as well add it to the track instead, saving you an extra step.
2. Buses can have automation lanes the same as tracks. Midi doesn't really have anything to do with it.
3. You can group controls but as far as I know you unless you want them to respond in the same direction you need to create a macro button for an fx chain, which basically means you can only group plugin knobs and not sends.

Sanderxpander
That would still require automating two faders which is what the OP asked to avoid. I think the only way is to insert a reverb directly on the track/bus and automate the plugin's own wet/dry balance (or create an fx chain macro knob if the plug has two separate controls for wet and dry level).

you don't need to automate the dry signal. only the wet (reverb fader)
however, now im starting to believe an Aux track might be a better idea
typical of me, changing ideas all the time. 
 
but similar to this:
https://www.youtube.com/watch?v=GzZD-Cj_X1g

Automating only the reverb signal means always leaving the dry signal in place which is different from hearing JUST the reverb. The way I understood the OP, he wants the option of having a 100 percent wet, no dry signal too. I could be wrong though.