Yes, but they don't stack.

PHB 206:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

For your specific case, having two Walls of Fire in the same space does not double the damage. However, if it was a Wall of Fire and a Cloud of Daggers, those effects would overlap and targets in the area would take damage from both.

To answer the title question, nothing prevents the second Wall of Fire from being cast on top of the first, but it wouldn't add extra damage.

The fireball will explode on the caster's side of the wall

There is no requirement that the caster see the target area (the spell does not specify "that you can see") but there is nothing in the spell that overrides the general rule that there must be a clear path from caster to target.

From the PHB:

A Clear Path to the Target

To target something, you must have a clear path to it, so it can't be behind total cover.

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.