I've seen people use 2/3 plants in their waterfarm, but if you're starting from seed I wouldn't recommend it
as you'll have roots all tangled together and if one of them is a male it'll be hard to separate those dead roots
from the live ones without damaging the live roots. Can probably be done successfully even if there is a male
though, just get something like hygrozyme to help break down the dead roots if you do have a male.
If they're both female and have similar flowering times should be good to go come flush time. Otherwise it
might also be hard to time out the flushes if their flowering times are too different, but again, probably doable.