We will be taking Tranzalpnie from Christchurch (Lv 8:15AM) to Greymouth (Ar 12:45PM). Pick up a car and drive to Franz Josef and arrive in Franz Josef around 3PM. Can I still do glacier walking or helihike that afternoon? or Should we stay one more night and try to do activities next day? Is it too much to stay in Franz Josef or Fox for 2 nights?

You can easily go heli-hiking in an afternoon. A guided glacier walk takes longer, but you can usually get one that starts in the early afternoon (not 3pm though, more like 1pm). I would stay two days, because the weather is very unpredictable. You will kick yourself if you arrive and its cloudy/rainy, and you only have one day. If the weather is great when you arrive, then see if you can book a last-minute heli-hike. Otherwise, wait 'till the next day.

I agree with Alf. That extra day might very well come in handy if the weather is poor. Arriving late in the day, then leaving the next day just doesn't give you much time to do anything, even if the weather is perfect.

IMO two nights is not too much anywhere in NZ.

Great! I think we will do 2 days in Franz Josef or Fox. Which glacier should we stay? It seems like Franz Josef town is bigger than Fox ... but is FJ better location than Fox for walking, helihike or any other activities that we might be able to do?

If you are doing a heli-hike, it really doesn't matter - both these glaciers merge upstream. The guided hikes are similar too. I have only been on the Fox Glacier hike - I'm told that the Franz Josef approach walk is a bit easier, but otherwise they are similar experiences. I would just stay wherever you think is best and then go to the closer glacier.