[quote user="clever_anthony"]What are the ideas to unite two slab elements?[/quote]
What you're referred to is termed a boolean operation on solids. Such operations include, for example, union, intersection and subtraction. Use the term boolean union when searching the API for your requirement to unite two slabs.
[quote user="clever_anthony"]What are the ideas to unite two slab elements in C# .NET?[/quote]
I can't give you an answer for C#, because I can find no relevant classes in the DgnPlatformNet documentation.
However, there are some examples delivered with the SDK that show how to perform boolean operations using the C++ MicroStationAPI. You'll find the examples in
..\SDK\examples\Elements\exampleSolids
Perhaps you can figure out from those examples what the corresponding C# classes should be?