[naturalgradient]

If you can go short on either one, you can create any bet via a weighted + leveraged long/short pair bet. I think bitfinex allows this.

[hxn]

I would not do it on an exchange for two reasons:

1) Counterparty risk. Bitcoin exchanges go out of business way too often.

2) They have enough money to manipulate the market.

[Bromskloss]

Could you clarify your second point? Would they manipulate the market in a way that is detrimental to you? Would it help that you were not their customer, but someone else's?