How can I use two microphones ??

Did you try picking a different device in the applications themselves? I know Skype can use a different input device then the default. However if your hardware treats the two inputs as one you cannot.

The whole reason for using jack sensing is due I think to the popularity of laptops where "private listening" is often required.

Pop the headset into the jack and the speakers and in-built mic are switched off

It's also useful for desktops where under the old system you would have to scrabble around at the back of the tower to switch from speakers to headphones.

With the availability of reasonably priced USB or PCI sound cards for those specialist reasons when multiple input and/or output are required I think the current situation is the best.

Another complication with today's audio is the dual use that jack sockets are used for with surround sound where the front Mic and Headphone jacks may be used for front and or centre speaker supply. This has complicated the software drivers

What you are able to use at the same time will depend on what options are in your BIOS and Audio Drivers. It would appear that you are able to switch off the jack sensing for the speakers (as you state you can use headphones & speakers together). It may be that this option is not available on your system, (the dual use of output devices is a lot more popular than inputs), or you may not have set the option.

One place to check is in the audio driver for the sound chip, you should see an option to allow exclusive use for audio devices, un-checking this may or may not, give an improvement

What do you see in the jack display in the manufacturer's control panel, with the two mics inserted, it may allow you to re-assign .

Last resort for your needs would be a jack splitter cable which would allow you to connect both microphones to the same jack socket, more advanced would be a small mixer panel which would enable you to balance the two inputs to your needs, and then feed the combined signal to the PC.

The most professional option (quality Wise), would probably be the use, as I suggested, of a USB sound card which would allow you to totally control the inputs. They are available on-line for a few dollars/Ponds