Can enter() selection be reused after append/insert?

No. The purpose of the data-join is to synchronize elements with data, creating, removing or updating elements as necessary. If you create elements twice, the elements will no longer correspond one-to-one with the bound array of data.

If you want two elements to correspond to each datum, then append a group (G) element first to establish a one-to-one mapping from data to elements. Then append child elements as necessary. The resulting structure is like this:

<g>
  <line class="line1"></line>
  <line class="line2"></line>
</g>
<g>
  <line class="line1"></line>
  <line class="line2"></line>
</g>

For example:

var g = svg.selectAll("g")
    .data([123, 456]);

var gEnter = g.enter().append("g");
gEnter.append("line").attr("class", "line1");
gEnter.append("line").attr("class", "line2");