I was laning bot as Vayne (with Nami support) vs Vayne (with Zyra support). Whenever one of us would have a silver bolt stack on something, and then the other attacked something nearby, it would reset the first person's silver bolt count on their target. I'm not sure if this could only happen when we were nearby, or what the range on it was. So for instance, if I hit her twice, she would have 2 rings. Then she hits me. The rings on her disappear, I get one ring.
repro steps: two Vaynes in one game should do it.