How can I merge a column from data frame to another column from the same data frame?
<p>So, I'd like to merge one columns in another column at the bottom. That's how my data looks like:</p>

<pre><code>    V1      V2       V3       V4
-75.6364 -33.3363 -68.6320 -35.7657 
-62.4546 -42.6754 -60.0532 -41.7773   
-53.1363 -48.3385 -56.0956 -47.4524 
-51.9633 -37.6143 -60.3062 -54.7817 
</code></pre>

<p>And I'd like to have this, merge V1-V3 and V2-V4: </p>

<pre><code>    V1      V2       

-75.6364 -33.3363 
-62.4546 -42.6754    
-53.1363 -48.3385  
-51.9633 -37.6143 
-68.6320 -35.7657 
-60.0532 -41.7773  
-56.0956 -47.4524
</code></pre>

<p>I was looking for at this page but I just found <code>cbind</code> and <code>rbind</code> options 
but do not offer me what I want.</p>