Просто добавьте float:right
к .box
классу
.box_LtR{
height:0%;
width:830px;
border-bottom:20px solid #c00;
-webkit-animation: increase 3s;
-moz-animation: increase 3s;
-o-animation: increase 3s;
animation: increase 3s;
animation-fill-mode: forwards;
}
@keyframes increase {
100% {
width: 1px;
}
}
.box_RtL{
height:0%;
width:830px;
float:right;
border-bottom:20px solid #c00;
-webkit-animation: increase2 3s;
-moz-animation: increase2 3s;
-o-animation: increase2 3s;
animation: increase2 3s;
animation-fill-mode: forwards;
}
@keyframes increase2 {
100% {
width: 1px;
}
}
<div class="box_LtR">
</div>
<div class="box_RtL">
</div>
Другое!
.box_LtR{
height:0%;
width:830px;
border-bottom:20px solid #c00;
-webkit-animation: increase 3s;
-moz-animation: increase 3s;
-o-animation: increase 3s;
animation: increase 3s;
animation-fill-mode: forwards;
}
@keyframes increase {
100% {
width: 1px;
}
}
.box_RtL{
height:0%;
width:1px;
border-bottom:20px solid #c00;
-webkit-animation: increase2 3s;
-moz-animation: increase2 3s;
-o-animation: increase2 3s;
animation: increase2 3s;
animation-fill-mode: forwards;
}
@keyframes increase2 {
100% {
width: 830px;
}
}
<div class="box_LtR">
</div>
<div class="box_RtL">
</div>