Yellowstone and Glacier in May

Glacier NP is generally still closed due to the snowfall that occurs during the winter and won't be fully open until late June or early July. Going to the Sun Road is the main road through the park and they are busy snowplowing it until late June or early July; suggest you contact the NPS folks to get a better schedule of opening dates, etc. You can see the park from the south if you travel US 2 and can probably get to the east side of the park at an earlier date but won't be able to travel the Sun road.
You will be able to visit parts of Yellowstone in May, but some areas will usually still be closed until late in the month; again I suggest you contact the NPS folks to get the best info on conditions.