multiple microphones to audio jack
I would recommend that you would not want to direct connect the two microphones as you show in the diagram. Instead it would be better to actually build or purchase a mic mixer box. 

A main problem is that many mics these days use a bias from the mic jack to power the sound detection element in the unit. In particular electret mics fall into this category. 

Alternatively if your mics are ones that produce an output signal voltage without a bias then you could consider the very simplest mic mixer. This would be a resistor in series with each mic output into the mic jack input with a third resistor to the low side of the mic jack connection. 



This type of resistor pad mixer does have the issue that it reduces the overall signal into the amplifier from each mic. This is one reason that most mic mixers as recommended above have a gain circuit for each mic input.