Suppose I have two different applications. One is in C#.net and the other is in VB.net. How do I combine these two applications?
If you mean combine them at design-time, your question has already been answered - Create a solution and either add existing projects to the solution, or create new projects and select the appropriate .NET language for the new projects.
If you want to combine them at run-time, there are far too many ways to enumerate - however the most common ways are to place both assemblies (.dlls, .exes) in the same folder, or place the independent assemblies in a child bin folder, or install the independent assemblies in the GAC (do read up on the pre-requisites and implications of doing this though).
If they are separate web applications, do read up on Response.Redirect & Server.Transfer to see if those can be of help.
As it says somewhere else, the more information you provide (but be concise please) in your question, the easier it is to give you a relevant response.