I have, in fact, used other music tools, including FamiTracker, which is also one of the best music tools ever. Also, I have an affinity for over-the-top statements.
No, the two players never have to be online at the same time to send their WarioWare DIY creations to each other. They merely need to add each other's Friend Codes first. Once both players have registered each other, there are two ways to transfer music:
a) On the DS version, you can access the Warehouse of any of your friends, and download any microgames, records, or comics that friend has stored. You can store up to two of each in your Warehouse for your friends to download.
b) On WarioWare DIY Showcase for the Wii, if any of your Wii contacts also have WarioWare DIY Showcase, you can send them your microgames, records, and comics. When you do, a yellow envelope will appear on their WarioWare DIY Showcase channel on the Wii menu. When they access the channel, they can then look at your creations and decide whether to keep them or not.
You never have to be online at the same time.