I think it bothers me because they're probably the two least productive employees we have. I want to address this issue - the first step to me seems to be "well, what do they do all day?"
I've had incidents in my team where there are people I thought weren't productive who were genuinely engaged in a million minor support tasks I never knew about.
But in any case, talking to them like adults rather than spying on them is the way forward.
Do you want the rest of your team to learn that it's important to be productive? Or would you rather teach them that it's most important to be seen to be working straight through for eight hours?
From my perspective, it's a cultural thing - we have tried to foster a two-way culture of openness and communication. We have 70 employees - 68 of which are okay with our environment of trust. What makes those two think they're special?
Are they ex-military and therefore trained to react violently when people sneak up behind them? ;)
EDIT: My ex-CEO tells a story of a former employee and Vietnam vet who warned him not to surprise him as his fighting reflexes might kick in, so it apparently can happen in real life.