Queries :: How To Filter One Field With 2 Combo Boxes

I have 2 unbound combo box's on a form 1 called cboclient1 and the another called cboclient2. would like to be able to filter field name client name twice by cboclient1 and cboclient2 here my sql from my query

Code:
WHERE (((Assets.Client)=Forms![report gen]!cboclient1)) Or (((Forms![report gen]!Cboclient1) Is Null));

This works perfectly for cboclient1 problem comes when I try and add cboclient2

I have tried

Code:
WHERE (((Assets.Client)=Forms![report gen]!cboclient1)) Or (((Forms![report gen]!Cboclient1) Is Null))AND(((Assets.Client)=Forms![report gen]!cboclient2)) Or (((Forms![report gen]!Cboclient2) Is Null));[/

This doesn't work at all....