Hi Joeanthonyluk
Thanks for contacting the Community.
1. Yes you can be working full time employed and also be Self Employed and filling in a self-assessment with HMRC.
2. Do you mean for this is my only job declaration? If so as you pay tax and National Insurance contributions on your employment and self-employment income in different ways. So the Self Employed work would not class as another job.
We have some helpful articles and guides on QuickBooks available here that may come in handy for any future queries and you can always ask here, we're happy to help!