i would say, keep them both within a single subform, and then play with subform's pagination.
This way, even if your text field needs to expand, it will not jump pages alone, rather the whole subform will shift, and the explanatory text will always hold it's position within the subform. Although within your subform you cannot have positioned layout (due to the expanding text field), so if you need to maintain some space between the explanatory and text field, then use margins and other spacing options.