It would be like having an altcoin called XT, that shares all your previous BTC. It's a bit of a mindfuck but that's exactly what it is. They may co-exist forever as long as some people are processing transactions. Other decentralized p2p projects have forked before, and of course one version ended up being the popular one, but the other one still exists, for example freenet.
It's a bit different with bitcoin though. If we think of bitcoins as tokens with a monetary value, there are some more issues with bitcoin's blockchain being forked than the ones any other p2p project would.
Given that coins owned prior to the fork will be spendable on both blockchains (able to spend them in one without changing the balance in the other after the fork), most likely that the value of the coins will be heavily affected. We'll have BTCs (coins owned prior to the fork, spendable in both blockchains), BTCxt (coins on the "Gavin fork") and BTCcl (Coins on the old 1MB block version). That's three types of coins for as long as both forks are sustained. (names are just examples)
People owning BTCs will be able to import their private keys in services or wallets using each blockchain and spend them separately. This brings up many possible scenarios. It's very likely that one chain will just day if we wait a little, but what if it doesn't? Will the big exchanges that will support both BTCxt and BTCcl? And the price? What about the price then?
It's very likely that we'll see a lot of panic if a fork is pushed by Gavin. There are so many possible outcomes that it's scary to even think about it.