The only mechanical difference are that both igniters are used instead of just one.
If you use Manual Start you dont get the following protections that would cause the start sequence to be automatically aborted.
Start valve failure.
Ignition failure.
Pressure Raising Shut Off Valve failure.
Hot start.
Hung start.
No N1 rotation.
So if you do a Manual Start in high OAT then you should be more successful at avoiding a hot start (I used to do that in the Middle East when carrying out multiple runs on the same engine) BUT you have to watch all the parameters and abort the start before any exceedence - if you get it wrong then the blame falls on you.