Yeah not quite sure what is going on here, but I have this too. In fact I think this is very common to not be able to queue two colonization tasks on the bar. However you can have it, so I am not sure what triggers this or why.  After you colonized one planet, you can colonize the other the next turn, it is not annoying.

Thanks for conforming. So it's a minor bug. Devs, please set fix on the "to-do list" - at the very end.