Is it normal for programmer to work on multiple projects simultaneously [closed]

Question

On a current job I have two projects to work on. First is very huge system and the second one is smaller but it also big (first project is being developed for 12 years, second for 4 years).

At first I was working only on first project and was trying to get used to it. Then I was moved to second project and tried there, so my knowledge about first project became shady. Now I have to work on both projects at the same time.

It's very hard for me because despite they both use java, they use different frameworks and the amount of code and business-logic to understand is very big so I really can't hold both that projects in my head.

Is it normal and I should get used to it, although my expertise became very squashy, what won't happen if I would work only on a single project? Or should I raise a concern or maybe change employer?