I converted X ETC to Y ETH, which I immediately sold (on Coinbase) for Z USD.
For IRS purposes, are these one sale or two?
If they're two sales, then the gain is the ETC->ETH, I think. (The ETH->USD has almost exactly zero gains, but maybe not quite exactly zero.)
Note that if Coinbase reports this, they'll report the sale of ETH (they don't know anything about the ETC). I don't want my story to look inconsistent with theirs!