Populate multiselect lookup control based on another dropdown in different entity form? For e.g. Acct entity has a form on which a single lookup of CDP is there and based on that CDP value, my values in multiselect needs to change

Is there any way that I populate multiselect lookup control based on another single selection dropdown value in different entity form? For e.g. 

1) Acct entity has a form 

2) On that form a single lookup of CDP is there 

3) And based on that CDP value, my values in multiselect lookup needs to change.

Any urgent help if appreciated.

So basically 3 entities are involved; 1 for form 2 for single selection dropdown and 3 for multiselect.