Welcome VIAN!
You certainly allowed to join multiple projects, we welcome you all the same no matter which project you are running! However, as has been said, if you're running multiple projects on one computer, then the respective work for each project will go down. This is of course saying that they are all on the same priority level. Within the "Low" priority, there are 2-3 "sub-levels" and some projects work at different sub-levels. This means that you may find that some projects "take over" others, even if they are both set to Low priority. Just something to look out for 🙂
Garry