I've got a downpipe thats got a second pipe from the wastegate welded back into the main pipe. Has anyone else got one like this? coz I wanted to know what would happen if you vented it to atmosphere?

Does the exhaust seal separate to this other pipe or will it just sound like its all falling off?

The exhaust housing does not seal seperately and will sound like it is falling off:D
You could however get your turbo exhaust housing modified to seperate the wastegate and turbine gasses....
Rob