If you stick to Love route (the three hearts locked) then you won't share Anna as it's supposed to be a vanilla romantic route, oppose to Love route, if you push Anna towards Slut route (encouraging her to be like one) only then you will share her.
To avoid sharing routes, make sure to to lookout for branching options and choose the one with solo events. Also ensure that you're choosing "romantic" events and options such as washing dishes to gain love points (Angel Anna pop ups instead of Devil Anna) atleast till you locked her on Love route (again, the three hearts lock system)
If by the guy you mean Ken, it's alright for now. You'll have the option to share Anna but you don't have to. You can bang his MILF Samantha so I won't recommend to go back/ ignore his quest.
Just a heads up, if you want additional scenes with Samantha then you'll have to share Samantha with Ken. You'll have solo scenes with her but to progress Ken/Samantha arc, you should share her, but the choice is yours.