I am pretty sure that you can get more than one grid to appear
I have seen a red colour (say for bars) and a green colour grid (beats) on my sequencer at the same time. Especially as when I select the circle for one , the circle of the other one selected does not get turned off.
But to be honest , most of the time when this occurs , I actually turn off the other one as it was not my intention to have both active. Did not really pay attention to it and am not sure if it only occurs in some situations only.
If Gil says that this is not by design , then it maybe that the previous color is not being removed. Noticed this when doing Dan's jokes that sometimes the colour of the timing marks did not correspond to the Lyrics timing track selected. In which case, I could not drop the "face" between the marks. After a few times of this occurring , I decided to do all the voice 1 first and then the voice 2 . in the jokes case this was not logical as the voices talk to each other ... for songs would be fine.
Edit: Just opened my sequence and selected both timing grids. Picture attached