You have to use varStatus
attribute to get the current index, and print elements from both arraylists at that index (this is of course assuming that both arraylists contains same number of elements):
<c:forEach items="${postitem.comment}" var="comment" varStatus="status">
<span>comment="${comment}" </span>
<span>comment_time="${postitem.commenttime[status.index]}" </span>
</c:forEach>