By rights the prepaid SIM cards available here in Singapore will allow you to roam but as aragno says, the roaming charges are likely to be substantial. You're better off having two separate SM cards, one for Singapore and one for Hong Kong, using the respective card to originate your calls and data from the place. Considering the brief period of time you will be here there isn't any point to doing anything more elaborate.

You might want to check out these threads in the Hong Kong forum:

http://www.flyertalk.com/forum/hong-...s-airport.html

http://www.flyertalk.com/forum/hong-...m-card-hk.html