free throw lane issues

[ysong]

Since it is generally acceptable to issue the warning first before a T or double T in this case, I wonder to which player the warning should be given?

If the warnings should be given to both players, then at what moment should they be asked to stay in their spots? at the moment when they are walking toward the same side or when they are at the different sides?

Please note that, if the warnings are for both players, the timing of the warnings actually mandate the postions they are going to stay to avoid the Ts.

Thanks.