using single COM port for multiple application

you cant share one com-port, so have have an architectual problem.

Somehow your apps need to know that the com-data is now from them. They can check that and than read or close the port and the nect app is to work.

Or you write a com-port reader app which reads all data and is sorting out to whom it belong. It can write it in a file, where it can be read from other apps.

Cant you extend the hardware, so you have more com-ports?

Serial ports can not be shared by multiple applications. Once a port is opened by one application, it is locked and trying to open it with another application fails.

The solution is to use a serial port splitter software. Such software opens a physical serial port and provides multiple virtual ports to be used by multiple applications. Google for 'serial port splitter' to find some products.

Hope this is still relevant...

Just have a look at com2com, tcp2com and hub4usb. They are all on SourceForge, just start from http://com0com.sourceforge.net/[^].
It is a Null-modem emulator, COM port to TCP redirector, and HUB for communications.
Using hub4com you can redirect incoming messages from your COM ports to multiple virtual COM ports quit easily. This way you can have the best of all three solutions mentioned.

Happy coding ;-)