Are you considering planting some digitalis nearby the Ajuga, or have you digitalis already there ?, the only effect it can have is that as the digitalis leaves bulk up they can starve the carpet of Ajuga that is near to the base of the digitalis, otherwise there is no problem, however Ajuga do better at the front of the border with digitalis further back, but having said that i have it growing between the digitalis on my allotment, i just take some of the leaves off so the ajuga can spread a little.

If you're wondering whether Digitalis has an allelopathic effect on plants growing nearby, no, it doesn't.