No way to do this currently. The coin-control feature is really "address control", with the intention of providing a way to decide which addresses get linked, not individual transactions. However, if you select an address and both Tx are different size, you can create a tx using the exact amount of the one you want to spend, and Armory will likely pick the correct one. If you choose "Create Unsigned Transaction" instead of just hitting "Send", you can review the tx to confirm it did the right thing.
If you want to spend a 1 BTC output but you are sending someone 0.4 BTC, then create a Tx with two outputs, 0.4 to the recipient, then use the address book to manually get a new address from the current wallet and send it 0.6 (or 0.5999 if you have a 0.0001 fee). You are basically doing Armory's job manually (for creating change), and Armory will realize it doesn't need a change output. It will also likely select the 1 BTC input.
Fair enough, I guess I'll use this until a specific selector is in place. Thanks.
