I don't think this is a good idea. Even if most of the questions for Spigot also apply to Bukkit as you claim, they're still separate programs that work different in some respects, so knowing how to do something in one of them might not apply to the other. This would be similar to merging C and C++ on Stack Overflow, or merging Windows 8 and Windows 8.1 on Super User. It's not because they're related to each other that they're the same.
After doing some more research, it turns out that Spigot and Bukkit are not the same. Bukkit is an API that allows mods to do extra stuff on your server. then there's CraftBukkit, which is the Minecraft server software with Bukkit baked in. Then there's Spigot, which is CraftBukkit with extra improvements.
Based on this summary, my original comparisons may be a bit wrong, but my conclusion isn't: they're not similar enough to warrant merging the tags. Spigot is a modded minecraft server, while Bukkit is an API to install mods into a minecraft server.