Congratulations on your acceptance at the THI!
Basically, you can only be enrolled at one institution of higher education (Hochschule or Universität/university) in Germany. Please contact the other Hochschule regarding this matter. They will tell you exactly what you need to do, if you decide to de-register there. We have no influence on this. Please also make sure to watch out for all the deadlines at THI regarding enrolment.
You must contact your embassy or consulate, if you have any questions about your visa for a different university. Unfortunately, we cannot help with this. THI has no influence and no detailed knowledge regarding the processes of the different embassies world-wide. They may vary in individual countries.