Have done this, all sorts of drama.

In theory, the one wire is the charge wire through to the battery (or starter, or wherever your main power feed comes from / to). The brown wire (gen warning light) you have to tape up, and you lose your charge warning light.

Doing it this way, you run the risk of alternator not firing up (if you don't run high enough revs, or muck around with pulley sizings).

There is a black rubber cover over a couple of other terminals on the back edge of the alternator. If you remove the cover, the charge warning goes to one terminal, and the other goes to the power supply (I just jumped it to the "one wire" post). This gives you an earlier field set up, and gives you back your warning light.

Check with a sparky, or google "alternator circuits". The wiring diagrams and descritpions of how they work let you figure it out pretty well.