Can I use a multi band compressor to tame the low end of an acoustic guitar, but only when the vocal sings?
I have most waves plugins and all of cakewalk,s.

The issue is that the vocal doubles the acoustic guitar line, note for note!, in some spots. Even in the same octave. To make matters worse, it's a low (bass range) vocal so the frequencies are piling up around 180.

Easy...
 
1. Clone the guitar track.
2. Use the Sonitus Multiband or LP MB as a crossover (i.e., ratio 1:1, high threshold, adjust the MB on one track so it's filtering only the lows; solo the band or mute the others. Copy the same plug to the other track but mute the low end band and enable the rest).
3. Place a compressor after the low band crossover, control with sidechain.

That seems convoluted (though perfectly workable). With FabFilter Pro-MB I sometimes dip the low end of the bass when the kick hits, you can simply side chain a single band. Can't you side chain the bands of LP-MB or Waves C6? I would assume you could, perhaps wrongly.

Yes - that all works.  Izotope alloy will also allow you to sidechain a single band in an mb

Craig's solution is elegant because you can do it with SONAR-bundled features alone. But it can certainly be done more easily with a side-chainable multi-band compressor or dynamic equalizer. FabFilter Pro-MB, Meldaproductions MDynamicEQ and MSpectralDynamics are three that I've used in this manner. I would think that Cakewalk's Linear Phase Multiband would be capable, too, since it features an external sidechain.
 
Oh, yeh. Forgot about Alloy.

I agree a solution with just Cake plugs is always nice. However, having to deal with two tracks for a single guitar part can cause other difficulties/complications in a project.

I just checked and it seems while LP MB only has a single side chain input, you can activate it per band. So I would say create a single band on your guitar track, in the range you want to duck, switch the threshold to "ext" and create a send from your vocal track.

Sanderxpander
That seems convoluted (though perfectly workable). With FabFilter Pro-MB I sometimes dip the low end of the bass when the kick hits, you can simply side chain a single band. Can't you side chain the bands of LP-MB or Waves C6? I would assume you could, perhaps wrongly.

The LP MB sidechains all bands at once. [Edit: although it can also be enabled for a single band]
 
However, he specified Cakewalk and Waves plug-ins, so I answered his question based on his constraints. Besides, if I was James, I wouldn't spend $200 to solve this one issue...I'd buy a Lifetime Update to SONAR. 

As I explained in a later post you can apparently create a single band with LP MB and switch that to external side chain, similar to Pro MB's implementation just with limits (that don't matter for this purpose).

BTW, "all bands", while technically true, is slightly misleading because you can create a single band with a set frequency range and not affect the other frequencies at all. Which happens to be exactly the application here.

Sanderxpander
As I explained in a later post you can apparently create a single band with LP MB and switch that to external side chain, similar to Pro MB's implementation just with limits (that don't matter for this purpose).

BTW, "all bands", while technically true, is slightly misleading because you can create a single band with a set frequency range and not affect the other frequencies at all. Which happens to be exactly the application here.

Excellent point! Thanks.
 
I'm still learning all the things the LP MB can do. I've  been spending a lot more time with the LP EQ. The mid-side option has been a godsend on a mastering project I'm doing.

Anderton

Sanderxpander
That seems convoluted (though perfectly workable). With FabFilter Pro-MB I sometimes dip the low end of the bass when the kick hits, you can simply side chain a single band. Can't you side chain the bands of LP-MB or Waves C6? I would assume you could, perhaps wrongly.

The LP MB sidechains all bands at once.
 
However, he specified Cakewalk and Waves plug-ins, so I answered his question based on his constraints. Besides, if I was James, I wouldn't spend $200 to solve this one issue...I'd buy a Lifetime Update to SONAR. 

The sidechain for the LP MB is enabled per band but can only have 1 source. So it sound like it would work in this situation which is the most common. All you have to do is create a band and turn ON sidechain, then send a signal to the plug-in and it will affect only the band with the sidechain enabled.