Hey Luke,
I’d love to have a way to switch between half, full and double time strumming while keeping the tempo constant. Here’s a short video explainer.
https://www.loom.com/share/20da3e47cc6b44eaa0bd678535b61ce6
So if I’m playing a melody at 70 bpm, the strumming can be doubled up without increasing the tempo to 140bpm. The problem with just increasing it to 140bpm is that you have to double up each chord in the arrangement to play the melody at the same tempo.
Jason, your post was very interesting.
I have a couple of waltzes (Louis Waltz and Rose of Avonmore for example) that I cannot get to syncronize with my fiddle playing.
I have been playing these tunes for many years.
I’ve checked various sources of sheet music, and/or modified SM, but I am still unable to play the tune in time with SM.
I wonder if what you’ve outlined, is the problem…?
Yes, very interesting idea, Jason! I’ll have to revisit it. Seems like maybe something that can be done with strumming pattern options, e.g. a “double-time strum”. 
John, generally with waltzes you’ll want to put two 3/4 measures between “barlines” in Strum Machine, as Strum Machine’s waltz-time barlines group pairs of 3/4 measures (it’s a bit confusing, I know). Feel free to start a new topic if you want more assistance with this – posting a video yourself might be helpful as well; you can upload one to YouTube (make it Unlisted) for free and then post the link here.