Multiple Price book for an Opportunity

I'm trying to build an customisation, which will allow me to link multiple price book with an opportunity. For that I have created a Junction object "Opportunity multi price book", which has two fields for look up on Opportunity and price book. After this I created another object "Opportunity products", which has look up on "Opportunity multi price book" and "Price Book Entry", but i don't see any option to lookup on "Price Book Entry" on New Object. Why that is so?

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Multiple Price book for an Opportunity