Hi Thevenin.
If both the units output DC, you can connect them together in series to increase the voltage (just make sure to get the polarity right, like you would with a pair of batteries). However, be aware that the units might not provide
regulated DC outputs.
If this is the case, the voltage will change depending on the load you apply.
(Originally the phone drew varying current levels from the unit when it operated or charged the batteries and this will alter the output voltage of the power unit) - I think this is what
Tommya300 is saying and
ymiky too, and they're correct.
The output voltage change is due to the power supply's internal impedance (think resistance) and occurs primarily due to Copper losses, Iron losses and heating within the transformer.
But if one of the units is DC and the other AC - and you connect them in series - you'll end up with a sizable AC component 'riding' on top (added to) the DC voltage and that could be BAD for your circuit -
Tommya300 says that too.
My advice is Test First!
Use a multimeter and a dummy load (resistors) to simulate your circuit load and verify the voltages you measure (I guess you don't
really want your circuit to be killed, do you?
)
Let us know how you go.
CJ