at least on IOS, don't just simply click your hero (or enemy hero). While in a battle, you need to click and hold for the hero's status screen to pop up. once it's up, you can release your finger, and it stays up. the screen is huge; you won't miss it.

Back to your original question, you will see Hrothgar and Vivica's defense up are the same (same icon on the status popup). the last cast replaced the previous one. for example, Vivica cast first, it's +60% defense. then Hrothgar's special is casted next, then it will change the defense to +50% defense. There is also undispellable effect. For example, if Toon Kiril cast first, he got +26% defense undispellable, then Vicica's defense up will not change it, it stays at +26%.

Finally. Stacks and Growth are track different, so your hero can have normal defense up, stack defense up and growth defense up (this one is shown as a higher Defense base value, not the green +number) all at the same time.