Frank,
Roll your dice as Lionel engines do not lash up well, this is not MTH who made engines that would work together with little issues. I think you need to put the TMCC engine in front as the Legacy engine will accomodate its speed with its front one. I have been told to make the Legacy engine as a TMCC engine in the cab-2 as another option.
The real drawback of having the TMCC one as the lead is in the sounds as the Legacy one has all the added sound features. Lionel never did their homework on the lash up issues. Way too many odyseey versions, motor versions to affect a smooth operation.
Regards, Ted