Here’s the intent: an Illusionist Wizard casts Phantasmal Force on themselves. Assuming they fail their own save, they proceed to make illusions of things only they can see. For example, a bridge to cross a chasm.
Then, they use Illusory Reality to make that illusion real. The bridge they see in their head now becomes real — but it also only exists in their own mind.
Let us assume the wizard can use Illusory Reality on Phantasmal Force. Then if the wizard’s party mates were to step on the bridge that they can’t see, would they fall?
Sure, this works. The bridge doesn’t ‘exist only in their own minds’, though. It becomes real:
When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real. You can do this on your turn as a bonus action while the spell is ongoing.
When you create the phantasm, only you can see it. When you decide that it’s real, suddenly everybody else can see it too. Note that convincing yourself to use Illusory Reality on illusions you have convinced yourself are real may be tricky, depending on your DM. Bridges are a pretty safe bet, though. Bridges totally seem like something I might have tricked myself into thinking was there when really it wasn’t there so that I could get to the other side. That and staircases. And it never hurts to give it a shot.