Although automatic formant tracking has a wide range of potential applications it is still an open problem. We previously proposed the use of active curves that deform under the influence of the spectrogram energy. Each formant was tracked independently and a complex strategy was required to guarantee the overall formant tracking consistency. This paper describes how the interdependency between formants can be incorporated directly during the deformations of formant tracks. Iterative processes attached to each formant are interlaced. We experimented two strategies. The first consists in partitioning the spectrogram into exclusive regions, each region affiliated to a given formant. The second consists in adding a repulsion force between formants that prevents formant tracks to merge together. It turns out that the second strategy is more robust and does not necessitate a complex control strategy.