What speaks against that? I would rather stop managing than not being able to play in a tournament I choose. And no there is no rule against it. Maybe you should read the code of conduct (don't know what that is) once again
The code of conduct is here: TT Code of Conduct - Approved
And this situation is referred to in paragraph 2.4 in particular:
2.4 With the exception of Grand Slams, Masters Series, Davis Cup and WTF (and any ATP 250/500 where there's only one tournament per week or one tournament of higher value than the others) a manager will not play in any event he manages. For the exceptions he will ensure that somebody is available to receive his/his opponent’s picks.