Is it possible to connect a reed switch to two separate arduino boards?
... How can i safely connect the same reed switch to two separate Arduino boards?
  (the boards are not connected between them in any way, they use separate adapters). 


If they are not electrically connected and if you wish to maintain the isolation you could use an optoisolator or similar. The reed switch could provide current via the LED in the opto isolator or voltage switched by the reed switch could apply voltage to a resistor connected to the LED. A reed could drive 2 x LEDs in two optoisolators this way this way if it had its own power supply or using the power supply from one Arduino. 

Placing the LED from an optoisolator in series with the reed switch adds a reliability hazrad - if the LED fails then the input fails on both Arduinos. Placing a few diodes or a zener across the LED such that Vdiode_string is slightly higher than V_LED_on will protect against LED failure as long as there is enough drive voltage. 

There are other isolating means available but an opt-oisolator is about as easy and cheap as any. 

If you do not wish to maintain isolation then by connecting the two systems' grounds together you could share a sensor. But, isolated systems seem with having in the context. One mode of attacking a security system is to use a TASER or electric cattle prod or similar. This tends to be poorly tolerated [tm] in most cases. If you wish half your system to live long enough to report the other half's death when under TASER attack you will need to rate the cross coupling isolators accordingly. One method I've heard of is to use a dummy video camera which is presented as first line target. It's health is monitored by the system proper. It's role is to act as a honey-pot TASER-attack sensor. [I won't tell you what country I heard about this from :-) ].