Honest Plans
You want a single estimate for each feature represented in the product backlog. Customers must weigh the different options to find the most important things to do next. You can't burden them with such issues as dependencies. Put the cost of developing that feature and that feature alone on every story.
If building this feature lowers the cost of that feature then you have already designed some shared code. Separate technical decisions from requirements decisions by estimating each feature independently and as if that is the only feature you will build. If developing this feature still lowers that feature's estimate then lower the estimate only after this feature is working. Remember that you are creating a shopping list for people without technical backgrounds.
