FSXA FIR sectors bisection in FSX Acceleration

In FS9 it was feasible to bisect an FIR sector, meaning that the lower part i.e. up to 11100m and the upper up to 100000m. The lower and upper using different frequencies. It worked fine in FS9. Yet, that doesn't seem to happen in FSX, as, if made so, ATC keeps interchanging those frequencies both for AI and user plane every 10 seconds or so.
Has anybody else noticed it and if there is any workaround to it ? Jim ?

Thanks for your time.
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