I can not specifically relate to a shellfish allergy, but I can to pork. I can NOT eat anything that has been cooked where pork has been cooked. So with that, I would I would say that it would be the same for shellfish.
I can handle pork with no problem, I just can NOT consume it in any way. That includes any other food that is cooked where the pork was, or with the utensils that cooked the pork. Any carry over of the pork, with the grease being the main thing being carried over, it will cross contaminate the other food.
I would not risk it myself.... unless you do what I do. I use two cookers, two sets of cooking utensils, and two tables. Every item is designated to what I am cooking.... hope this helps.
Smoke-N-My-I's Competition BBQ Team
2 Chargriller SuperPros with SFB's(retired)
2 UGLY drum smokers
Weber 22 1/2" & 18 1/2" Kettle
Super fast RED Thermapen
Screaming fast ORANGE Thermapen
KCBS Certified Table Captain