Combining two phantom power microphone capsules
The AKG PT40 Pro manual, page 17, states

You can use the PT 40 PRO bodypack transmitter with both dynamic microphones and condenser microphones operating on a supply voltage of approx. 4 V. You may also connect an
electric guitar, electric bass, or remote keyboard.

This is good as you now have several options. Note that the guitar input has a mini-XLR connector so you need their MKG L guitar cable to connect.
Condenser microphones
On page 19 we read:

Audio input pinout:
Pin 1: shield
Pin 2: audio inphase (+)
Pin 3: supply voltage
A 4-V positive supply voltage for condenser microphones is available on pin 3.




simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Condenser microphone bias circuit.
Your mic should have a circuit something like that of Figure 1. R1 provides the required bias current. Since the manual says that 4 V is available on pin 3 that suggests that the resistor is in the mic capsule. In this case you could parallel several complete capsule circuits, connecting them all together at the right hand side of C1 and feeding the combined signal to your transmitter input. I'm not sure if they'd interfere with each other.
Mini-mixer



simulate this circuit
Figure 2. Mixer.
Figure 2 is a simple inverting summing op-amp mixer. The output is the sum of the inputs and the circuit configuration prevents interference between the inputs.

The gain is set to 1 (actually -1 as it's inverting) by the ratio of each input resistor to R4, the feedback resistor.
No capacitors are shown on the inputs as it is assumed that these exist already on the microphones.
The '+' input of the op-amp is biased to half supply, 4.5 V, to allow the signal to swing high and low. The output is biased to the same voltage so a decoupling capacitor is used to block DC getting to the next stage.

If using this circuit I suggest you feed to the guitar input. If required you could increase the gain of this circuit by increasing the value of R4 to 50 or 100k.
Why am I helping an accordion player?