I want to roast potatoes and fennel, but do I roast them in the pan with the turkey? In a separate pan?....after the turkey is out of the oven?

Either way is fine.
If you have vegetarians coming, roast them separately, serve separately...so they are not full of meat fat and flavor.
Otherwise your choice. Potatoes take about an hour, fennel maybe a little less. So if you are doing them with the turkey, remove after about an hour, and rewarm later. Or add to the roasting pan (cut up) for the last hour.
Make sure they are in some fat or liquid so they don't roast too dry or burn.

hi robyn! they'll roast much faster than the turkey, so i'd say to roast them separately—and if you want some of the flavor from the turkey, you could wait for the turkey to come out of the oven, toss the chopped potatoes and fennel with some of the turkey drippings, and then put the vegetables in the oven while the turkey rests.

I would go separate pan also, but your veg is not above some turkey drippings if you want to drizzle/toss some in there once you have it. :)

As Julia Child always taught us, don't crowd the pan! Vegetables should be roasted at a high temperature (i.e. higher than you'll cook your turkey) so use a separate pan and cook them when the turkey's out of the oven.