The Xceed Encryption Library v1.1 ActiveX control supports both the Single-Threaded Apartment model (STA) and the Multi-Threaded Apartment model (MTA). The entire library's code is threadsafe, can be instantiated multiple times in order to perform multiple memory or file encryption, decryption, signing or hashing operations at the same time, and is designed to use very little memory per instance – so your machine won't run out of memory when you're running dozens of instances at the same time.
Yes: two or more instances may not write to the same destination file when using the WriteFile or ProcessFile methods. Other than that, running multiple simultaneous operations involves having multiple instances, with each instance running its own operation independently.