I have been playing with the online mixtape creation gizmo Dragontape
Although there is limited control over the blends between tracks, it’s got a great interface and is as good an online video mixer as i have found.
I suspect i shall become mildly obsessed…
http://www.dragontape.com/dragonplayer_v2?tapeid=5306024&autoplay=0&fullscreen=1
Re# 
‘ would like to collab – but how? – I haven’t understood it … this would be my part: http://www.youtube.com/watch?v=cGSn-G2Peus …
best
CYT
Hey, be great to collaborate with you and fantastic choice. I follow that YouTube user, is it you?
Remixing teh internets is locked, the one open for sharing is called ‘collaborate mixtape’. If you enter the URL in the box at the top it will find the video and you’ll be able to drag it to the timeline. Click on it in the timeline to trim start/finish points to get the mix right. You can also trim the track before. Mixes limited to a 10s audio fade, video just shows whichever is ontop, no fades.
Let me know if you can work it out.