What would Bitcoin be like with two block chains?
Most other crypto coins is more or less what you describe.

The bitcoind code has build-in "checkpoints" (block height x has hash y), but a new blockchain would anyway use a "new" ( / forked ) client, so you could just remove these checkpoints.

However the 21 million bitcoin limit will never be a problem. Today each bitcoin can be divided down to 8 decimal places (0.00000001 BTC), should we for some weird reason need more bitcoins, then this limit can be changed.