i'm wondering how much the system can resists to power overload and how balancing between the 2 power input works on the ccr2004-16g-2s+. I have for now plugged it to two APC UPS but i'm thinking to only keep one and plug on of the input directly on AC (behind a differential switch sensible to 30mA). while the other would be on battery. thoughts?

Generaly redundant power supplies feed common DC power bus and load transitions between working power supplies smoothly as song as power supplies provide equal voltages (which is not always the case with Mikrotik devices with multiple powering options). Point being that as long as one of supplies provides power at every moment, switching off/on of power supplies does not affect device.

Having written that: if both mains and UPS provide power, then device will probably draw power via both inputs (ratio is then depending on state of power supply circuitry). If, OTOH, you're thinking of switching between power sources ... you may be risking damage to power supplies (due to inrush currents) and potentially device itself (if switch-over tine is long enough to drain energy from capacitors in power supplies).