Merge Spec Bars
If you have dual talent specialization, Merge Spec Bars will keep your two sets of action bars synchronized.
If you like, you can still specify some of your bars to maintain different actions, just like normal. For example, you can have your main bar have different spells for your two specs, but your lower-right bar can stay synchronized so that your potions and items don't change.
To configure the addon, you can open Game Menu > Interface > Addons > Merge Spec Bars, or type '/msb' or '/mergespecbars'.