different rhythms of notes on the same staff?

So, I'm creating a piano arrangement, and for the bass clef with the left hand, I have some half notes. Now I want to put some eighth notes above that, but have them played by the same hand. but whenever I try to put eighth notes above that, the half notes automatically turn into eighth notes and it won't allow me to put a half note for beats 1 & 2 and an eighth note on the off-beat of 1. (This is in 4/4.)

I've tried looking in the handbook but to no avail ;^;