I believe having more than one bar can make you windy, and they do have slightly more carbs so it's possible it could bring you out of ketosis.
I think if you feel desperate to, i's worth talking to your counsellor. I had 2 bars a day on Lighter Life when I was travelling, because mixing shakes wasn't possible and there are no tetra bricks on LL. Didn't have any adverse effects on me, but it's different for everyone.
You could try it and see if it has an impact on your losses, and decide from there.
I think it's better to have 2 bars and a shake, than to fall off the diet, but it's a totally personal decision.
My counsellor recommended it to my friend because she just couldn't stomach the packs at all, so I guess some people do it.