Yes they can. If you are on foot, you do not gain the +1 defence that the shield would normally give you, but you may still use the shielding rule and may fire your bow as normal.
However, if a model is mounted and it has the Expert Rider special rule, the rider does gain the +1 defence, and so there are no penalties to using either the shield or the bow, except for the fact that mounted models cannot shield. If the rider is dismounted however, he behaves like a normal infantry model, and so loses the +1 defence that his shield initially gave him.
In short, infantry and cavalry models without the expert rider special rule do not gain the +1 defence when carrying both a shield and a bow, but a mounted expert rider does.
Hope this is clear/ helpful
