Hi, I'm going to Hangzhou in end March.

Hi, considering the sightseeing duration, time shall be a little tight for your one-day trip between the two places. I suggest you go there on the first day and stay at Huangshan for a night and then travel back in the next morning.

It is easy to find a local tour both in the two places. Anyway, for convenice, you can book a tour in advance. And I recommend TravelChinaGuide to you. Here is their email address: travelchinaguide.com|tours
Send them an email to tell your requirements and they will arrange all for you.
Hope help and wish a good trip!