questions about 2nd index on one column

I have a bit strange question. I am trying to figure out how to avoid table
locking while creating an index through Django (1.5.1) in Postgres 9.4.7

Django 1.5.1 does not support concurrent indexing. So my thought is to
first create a concurrent index using SQL prompt.
Then try to update django model to add index, which will effectively create
2 indexes on same column.

So my questions are:

If I create a 2nd index on one column,
1. Does postgres scan entire table to create index from scratch or does it
simply copy the first index?
2. Does postgres lock the table to create this index?

thanks