Here, two objects A and B are moving relative to each other. I use the Einstein velocity addition formula $v = {v_1 + v_2\over 1 + {v_1 v_2\over c^2}}$ to calculate the relative speed between A and B, where $v_1$ (for A) and $v_2$ (for B) are as measured by each observer in their own inertial frames and are as relative to the observer (say there is an observer C, then $v_1$ and $v_2$ are measured by C relative to C). The question is will the observers all calculate the same value $v$ in their own frames? In particular, will A and B agree on the same value $v$? Let's say one calculates $v = 0.1c$, will others also get $0.1c$?
Another thought is if A and B are stationary to each other as measured by C, will another observer D also see A and B stationary to each other?
Update
Is this question related to the principle of relativity, i.e.(copied from wikipedia), if a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K.
So in the above exmaple, if C sees A and B are stationary to each other, but D doesn't agree, wouldn't this be a violation of the principle of relativity?