Can I use the Caestus with a shield on?
Yes, you can attack with both your sword and cestus.

The wording in the rule book (pg. 22) is as follows:


  During an activation, a character can make up to one attack
  with each weapon they are holding in their hand slots.


The special ability of the cestus explicitly states you can use it to attack even when it is in your 'backup slot'. There is no qualifier regarding how many weapons are equipped in your hands. This special ability on the card overrides the general rule on attacks (hence the reason it's a special ability)

This means you could have a 'two-handed' weapon equipped and your cestus in your backup slot, or two single handed items and your cestus in your backup slot.