Hi,
It probably won't tamper too much, but the real question is why do you need to do this?
Is it one-off case of convenience (e.g. all day meetings/day out)
or
just because you want to?
If it is just because you want to, then its probably not a good idea because part of being on LL is learning self-restraint around food - being in control of it, and sticking to some rules/limits... Its the psychological aspect of the program...
My advice from being on LL for 40 weeks is, don't do it...

Even for convenience, you can always add the milkshake sachet powder into an empty bottle, and then fill it up with water later when you need to...
If you start breaking the rules now, its a slippery slope to braking more and more rules...
If you have already had two bars in one day ;-) then don't worry - it will probably have almost zero impact on your weight loss - but beware of becoming more prone to breaking the rules in future...
just my advice
