I am using Installshield 2015 with Visual Studio 2013. I created a setup.exe for my application. In order to start my application, user needs to run SpeechPlatformRuntime_x86.msi or SpeechPlatformRuntime_x64.msi.
How can I add these msi file into my project so after installation finishes, it automatically run msi file?
These sound like they provide dependencies. Since you deliver a .exe file, I would suggest creating a prerequisite and including it in your project. This will result in them being installed first, and your main .msi (or InstallScript) project contents will follow.
(If you delivered only a .msi file, you would probably have to document your dependency. You could look into the Chained .MSI Packages support, but it's not really that well suited for handling dependencies.)