Company E was primarily from Montgomery County, so that is perhaps the best choice.

However, we also know that at least one member of Company H, which was primarily of Butler County and that you had originally asked about, was a resident of Montgomery County. So that would be plausible, too. Note that both Company H and Company E were taken in to Confederate service in Greenville, Butler County. You might even want to create a scenerio for this that would add an interesting story of the cousins getting left behind in Montgomery and an adventure in getting from Montgomery to Greenville.