Best way for edit more objects at same time

This is a UI or view or presentation level issue, not a business model issue. In other words, your existing business object model doesn't need to be altered. What you need is a viewmodel that manages the interaction between the one UI control (your textbox) and the set of selected items in the collection. I've done this for a client or two over the years, and it is not a terribly hard problem to solve as long as you recognize the need for a UI-specific viewmodel object that handles this particular type of interaction.