I do not have running ofp now, so I can not test it, but...
1:
there is a missing " after the @ operator!!!
2:
anyway you do not have to work with alive command here.
you should simply count the units of the groups. If any group member dies the headcount will decrease by one...
So use this:
bla
bla
bla
@ (count units _grp1) + (count units _grp2) < _all /2
Hint "it works"
3: If you it still does not work, I can imagine you do not have to use 'units' word. Maybe the arguments are made without 'units'...it is not sure