Do Toledo and Segovia in one day? No, you would not be doing either city any justice, they each deserve at least one day, or a day and a half.
Toledo has so much to see and experience that a half day trip would mean a 2 hour stay, and you cannot do justice to the cathedral in 2 hours. In my opinion.
Segovia, can be done on a half day tour, but, I recommend a full day,particularly if you want to take a leisurely walk from the Roman aqueduct past some of the most interesting medieval churches in Spain, then climb to the Castillo, with its marvelous view of the valley below.
I would recommend, if you only have one day, to go to Segovia in the morning, lunch there, then onwards to El Escorial, and see a truly beautiful museum. Or, go to the walled city of Avila, where St. Theresa de Avila was born and established her ministry.
Salamanca is so charming and centraly located that you can do it in a day, but, only if you take the bus, because the Renfe train schedule is limiting. They have the second most beautiful Plaza mayor, second to Madrid, and lively street scene as well. The calle de los Libreros is atractive to all booklovers. The medieval and renaisance cathedrals are outstanding. Then you have the University of Salamanca, one of the earliest in Europe. The historical center is easy to navigate and most of the monuments are all easily within walking distance, particularly if you are young.
Good luck and enjoy your trip.
_________________________
The world is a book, and those who do not travel read only a page.
--St. Augustine (354-430)