Question about Philips MAS LEDtube

I bought two of these tubes to replace the two old fluorescent tubes in the double socket in the bathroom. I replaced the old tubes & starter and installed the starter & LED tube supplied. But now I have the problem that one of the lamps is noticeably brighter than the other and even stranger is that every time I switch the power off and on again, the tube that is brighter changes. But one of the two is always brighter/darker than the other. I think the two tubes are connected in series, because when I take one of them out, the other one doesn't light up either. Does anyone have any idea what I have done wrong?