Yes, you can log in and use the same account on different machines at the same time, but this is usually subject to OnlineConvert's terms of service and geographical restrictions.
If you try to log in and use the same account on machines in different geographical regions, you may encounter service unavailability or restrictions. This is because some online services may restrict access to their services based on geographical location, copyright restrictions, technical limitations, or other legal requirements.
Therefore, although you can technically log in to the same account across machines, in order to ensure the best experience, it is recommended that you understand and comply with the relevant policies and restrictions of OnlineConvert before trying. If you need to use it across regions, it is recommended to contact the website's customer support for more specific information and guidance.