user100284: I can create addresses with private keys and import those keys into my wallet to receive their unspent outputs to later spend.
My question is split into two parts:

What exactly happens when you import a private key? Is that private key now stored and its corresponding address now tracked by your wallet?
A new transaction is signed with a private key. Therefore, if I have 1 BTC on one address and import its private key, and 1 BTC on another address and import that, how is it I can now spend 2 BTC in one transaction, which key was used to sign this new transaction?


user13866: 
Yes
Both. Each of the two keys is used to prove control over the corresponding UTXO of  the two UTXOs used as an input to the transaction


100284: <p>I can create addresses with private keys and import those keys into my wallet to receive their unspent outputs to later spend.</p>

<p>My question is split into two parts:</p>

<ol>
<li>What exactly happens when you import a private key? Is that private key now stored and its corresponding address now tracked by your wallet?</li>
<li>A new transaction is signed with a private key. Therefore, if I have 1 BTC on one address and import its private key, and 1 BTC on another address and import that, how is it I can now spend 2 BTC in one transaction, which key was used to sign this new transaction?</li>
</ol>
 13866: <ol>
<li>Yes</li>
<li>Both. Each of the two keys is used to prove control over the corresponding UTXO of  the two UTXOs used as an input to the transaction</li>
</ol>
