In older version 2013/2015 i use this code to achieve multi entity lookup, but code is broken in 2015 Update 1 any solution in new version.
//Setting the lookup type
document.getElementById(LookUp_Logical_Name + "_i")
.setAttribute("lookuptypes",
"<EntityTypeCode of Entities to be shown in the look up>");
//Setting the lookup name Ex :- account:1:Account,contact:2:Contact
document.getElementById(LookUp_Logical_Name + "_i")
.setAttribute("lookuptypenames",
"[<Logical name>:<EntityTypeCode>:<Schema Name of Entity>]");
//Setting the lookup icon Ex:-/_imgs/ico_16_1.gif:/_imgs/ico_16_2.gif
document.getElementById(LookUp_Logical_Name + "_i")
.setAttribute("lookuptypeIcons",
"/_imgs/ico_16_<EntityTypeCode>.gif");
// default type - Account entity To set the default entity
document.getElementById(LookUp_Logical_Name + "_i")
.setAttribute("defaulttype",
"<EntityTypeCode>");
https://mscrmmindfire.wordpress.com/2014/05/20/crm-2013-manipulate-look-up-to-show-multiple-entity-record/

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