I've finished my tests. Here are my findings:
Each relay draws about 90mA (according to my measurements), so theoretically it is possible to use as many as 2 relays simultaneously, given that Arduino's Vcc pin can source 200mA max (refer here for your board's limitations). However, if there are additional loads connected to the arduino you might exceed this budget. In this case it is possible to power the relays with a dedicated power supply, and then only 2mA will be drawn from the arduino (enough as to light the LED in the octocouplers).
This alternative power source can be attached to the left group of pins, and it will be used to switch the relays. Again the source must be able to provide about 90mA per relay. I made my tests with a 5V 350mA phone charger.
The alternative power source must be connected in addition to the arduino pins on the right block. Thus, Arduino Vcc and GND should remain connected to the right group of pins, and also IN1, IN2, etc to Arduino's digital output pins, while the secondary power source must be connected to JD-VCC and GND pins in the left, after removing the jumper. I've read that in this case Arduino's GND should not be connected. This is actually nonsense, since both GND pins in the module are interconnected (you can see this by looking at the soldered paths in the back side) and I think it is bad practice to leave the arduino's Vcc floating in the module, instead of having a common GND for all the power sources. When using the alternative power source, it is possible to connect the right VCC in the module to just 3.3V.