Hello,
Well, the webparts in Kentico
CMS Portal engine model are a bit different thing from default ASP.NET webparts. Therefore, the best and the most reliable way in Kentico CMS is incorporating multiple controls within one webpart and manage their data and properties in the code-behind of this "shared" webpart. Please see more info about developing webparts in our Developer's Guide:
http://devnet.kentico.com/docs/devguide/developing_web_parts.htm.
Other option would be managing data of the webparts on the page using our
API, more precisely CMS.GlobalHelper -> RequestStockHelper class.
Hope it sheds more light.
Best regards
Ondrej Vasil