Stuff the birds:
Place an onion wedge in the back of the cavity. Then stuff as many baby carrots and 3 or so pieces of celery into the cavity. Squeeze lemon quarters in when halfway full. Keep quarters for roasting bag. When stuffed, use another onion wedge to seal the end.
Next, liberally coat with salt, pepper and poultry seasoning. Carefully, rotate and season the birds to cover entire bird.
Prep the bag:
Prep the bag for baking according to the instructions on the box. (Most baking bags require shaking flour on the inside to prevent the bag from bursting during baking. Read your bags instructions because it won't be pretty it the bag erupts in the oven. Seriously.)
If you don't have a roasting pan you can use a sheet pan and an aluminum foil "snake" to support your bag while baking. Either way, place the bag vertically in the pan and open it up.
Place the remaining onion, celery, extra bit of carrot, and lemon wedges inside. Squeeze the other two pieces of lemon wedge over the vegetables. Place all squeezed wedges in the bag.
Remove from oven and allow to rest for about 5 minutes to cool. If you want to save the juices from the bag. You should do this before opening the bag. Otherwise, to remove the hens just cut away the top of the bag.
The juices in the bag are great for adding to stock or preparing gravy. To reserve the juices, carefully transfer the pan to your sink. The pan will be hot. Place a large bowl in the sink and maneuver the pan over the bowl so that the hens are supported by the pan.
Using kitchen shears or a knife pierce the bag and empty the stock into a bowl. Allow the stock to cool in the fridge, then pour into a freezer container and freeze; best within 3 months.