Well, Pg. 462 in the Nagash rules has a pretty convincing argument if you guys are willing to whip out Undead Legions on your friends.
"Last but far from least, we have included a brand new army list that you can use for combined Undead armies of Sylvanian (Vampire Counts) and Nehekharan (Tomb Kings) troops. It allows you to take units from the Vampire Counts and Tomb Kings army books together as a single force. blah blah blah.. Along with all of the other sections of this book, it is intended for both with the scenarios presented herein, AND in any other games of Warhammer you would like to use it for."
Looks pretty clear to me :X
"Last but far from least, we have included a brand new army list that you can use for combined Undead armies of Sylvanian (Vampire Counts) and Nehekharan (Tomb Kings) troops. It allows you to take units from the Vampire Counts and Tomb Kings army books together as a single force. blah blah blah.. Along with all of the other sections of this book, it is intended for both with the scenarios presented herein, AND in any other games of Warhammer you would like to use it for."
Looks pretty clear to me :X