Unlucky mate.
I've just gone through similar. A mate of mine left his car at my place while he went away on holiday. His and my car were parked facing one another.
Got up Monday morning and his tyre is flat. Thought maybe he had a slow puncture. I got in mine to drive and noticed the steering felt funny , got out and lo and behold I've got a puncture as well. Hmm.
Inspected both tyres and someone had slashed/ stuck a knife into the tyres on both cars. It couldn't be co-coincidence as incision was exactly the same width and on the sidewall.
At least they were kind enough to just do one each
so that's only a new tyre for each car.
Cheers
Ben