There are no technical limits to how many times you can log in using your login data - just enter the data in all the newsreader programs you are using across your devices. If you own two computers, you can use your account on both of them, you do not have to sign up for two contracts.
Please note that downloading from the usenet usually maxes out your internet connection, so we do not recommend to have several clients running at once.
Apart from this it is of course not allowed to share your login details with anyone. Please make sure that your username and password are secure and not known to third parties.