Simultaneous means at the same time. Simultaneously is an adverb which means it describes an action being performed by multiple people/objects at the same time. They whole class simultaneously...
Because I'm currently downloading email in one application and video in another, I'm using two connections simultaneously. Simultaneously, which means occurring at the same time, can be used like...
It describes multiple events happening syncrimoniously.
The snipers made simultaneous shots to avoid detection.
The friends called each other simultaneously and left each other messages.
