In vizzy, when you make a program to stage the second craft or booster, add a block to the program that renames the booster or second craft to the original craft + 1. Ex. Set variable (booster) to [(Craft ID of (Craft name)) + 1] then when the craft separates, broadcast a message named booster with data (booster)variable and in the code of the booster, add Receive booster with data. then under it put the block: set variable (booster) to (CraftID of (Craftname)). Then add the block: switch to craft (booster)variable under the message block with 1 second of time between each block ideally.
Hopefully, this is understandable, feel free to ask questions about the way things are worded or any other issues.