You have three animation timelines.
1, main timeline
The elements are the same objects regardless of timeline that tries to control it’s properties to perform an animation.
Look at the timelines as a tool to organise chunks/layers of operations in a single scheme.
You first need to set the text opacity in the main timeline to 0.
Then in each B1 and B2 timelines make sure the opacity matches what it is in the Main time line (0) for the start keyframe and the end keyframes are 100.
loosely speaking… In most cases like this you will always set the main timeline first as the other timelines take their start properties for elements from this. ( In some cases you will override this, but thats a whole other ball game )