On many places in the books there are mentions to periodical stops that voidships must make from the warp travel.

I take it that they only translate and remain still until their navigator rests enough and their gellar field generator cools down/is repaired so they dont need to re-chart course.

But how many of these happen in a 5 days travel? How much time inside the ship has to pass before they need to drop back to real space?

That's interesting information for navigators that want to do something else than navigating during these travels, or for a matter of finding new worlds or star systems by mistake.

navigators run in shifts from what i heard. pc navigator may be the head but he is rarely the only one. so you may not have to (may be thinking of calculated jumps)

Ships without a navigator, such as commercial transports, need to stop every few light years to reset their bearing. Or when travelling in an area where you can't see the Astronomicon. You need to periodically make sure that you are headed in the right direction.

Even for crossing the Maw with a fully warp capable ship (using navigators) you need to make some stops.

Even for crossing the Maw with a fully warp capable ship (using navigators) you need to make some stops.

True, but whilst relatively short, the Maw is anything but 'normal' travel - it's shooting the riptide between two warp storms. Short of temporary storms, it and Hell's Slingshot are probably two of the scariest bits of warp navigation in the imperium.

I was just thinking of a copilot seat for a Navigator and the other navigators could sit in and watch and help lookout for stuff.

Is there a ship mod for this? and if so in what book?

Not mentioned in the Navis Primer I believe.

In all the WH40K fluff I've read even when vessels are acknowledged as having Navigators it tends to be a one-at-a-time thing. I suspect that given how everyone interprets the Warp differently two Navigators wouldn't necessarily be compatible, so each Navigator will perceive the Warp a certain way and probably want to steer the ship a certain way given what they're seeing. The more you try to add simultaneously navigating probably the less accurate it would be as they would be in competition, not support.