I have 2 sister Guinea Pigs from the same litter. I've noticed that they will look at each other and shake their butts at each other. They'll also start purring at the same time. I have no idea what this is and need help!
It sounds like rumble-strutting and it's a normal thing to decide who is the dominant piggy. Girls and Boys both do it - although it's always a good idea to double-check yourself that you have 2 girls! My boar does it to try and impress the ladies but when he got a new little cage mate she was also doing it right back at him to say "hey, don't mess with me big fella!" - it made me chuckle because she was small but feisty! It calmed down after a week or so when she realised he wasn't a threat but he still 'revs up' like a motorbike before he rumbles out of his tunnel showing his best dance moves.
If they've been together since birth do you know how old they are as their age might have triggered them to start doing this. Or maybe they have been moved into a new cage/enclosure recently and this has triggered it. Make sure their environment has enough space for them as they are growing up - there are some links up the top and if I can link one here... Cages And Hutch Guide there we go I think that's worked. Cages which are too small can cause no end of falling out as pigs can't get away from each other. And well done getting 2 - they get very lonely when there is just one but in the States this still seems to be quite accepted - maybe because they look a bit like big hamsters which do prefer to be alone. But piggies are a herd species, like people!
Although it is normal dominance you have to keep an eye on things to make sure it doesn't escalate into aggression, nipping, or bullying behaviours like top-piggy not letting under-piggy near the food bowl or water bottle (which is why it's convenient to have more than one!) Boars can have full-on fights which means they have to be split but I don't know that sows usually have this problem. Again, have a look at the guides for new owners