Both devices should charge.
If you can't pedal enough to keep the long march charging the N900 will still charge until the Long March is flat (which is going to take a while).

Edit...
Just tested this and you can't charge a device and the long march at the same time.

Although I can't test it 100% if you supply the long march with power the outputs disconnect.
When you stop pedalling the input voltage will drop and the long march will start charging the N900 until pedalling resumes.

You could therefore ...
...have a switch on the lead supplying the long march to kill the supply and start charging from the long march.

...create a switching arrangement that will switch the supplies around so you can keep both charging or only the long march charging with the above arrangement.