user42058: Is it possible to create a 1 of 2 multi-sig address in which only one of the keys is time-locked and the other can sign any time?

user10397: 
only one of the keys is time-locked

Keys aren't time-locked. Transactions are. However, Bitcoin now has an opcode for this purpose: OP_CHECKLOCKTIMEVERIFY.
Essentially, it forces the transaction to have a lock time further in the future than its argument, otherwise the transaction is invalid.
In your case, the script would look like this:
OP_IF <this pubkey can spend at any time> OP_CHECKSIG OP_ELSE <block number> OP_CHECKLOCKTIMEVERIFY OP_DROP <this pubkey must wait until block number> OP_CHECKSIG OP_ENDIF

The spend-anytime key can redeem the transaction with
<Sig> 1 <above script>

The other key can redeem with
<Sig> 0 <above script>

but only after the given block number.

42058: <p>Is it possible to create a 1 of 2 multi-sig address in which only one of the keys is time-locked and the other can sign any time?</p>
 10397: <blockquote>
  <p>only one of the keys is time-locked</p>
</blockquote>

<p>Keys aren't time-locked. Transactions are. However, Bitcoin now has an opcode for this purpose: OP_CHECKLOCKTIMEVERIFY.</p>

<p>Essentially, it forces the transaction to have a lock time further in the future than its argument, otherwise the transaction is invalid.</p>

<p>In your case, the script would look like this:</p>

<pre><code>OP_IF &lt;this pubkey can spend at any time&gt; OP_CHECKSIG OP_ELSE &lt;block number&gt; OP_CHECKLOCKTIMEVERIFY OP_DROP &lt;this pubkey must wait until block number&gt; OP_CHECKSIG OP_ENDIF
</code></pre>

<p>The spend-anytime key can redeem the transaction with</p>

<pre><code>&lt;Sig&gt; 1 &lt;above script&gt;
</code></pre>

<p>The other key can redeem with</p>

<pre><code>&lt;Sig&gt; 0 &lt;above script&gt;
</code></pre>

<p>but only after the given block number.</p>
