is this possible to use multiple urlfilter lists in web filtering on fortigate? I need to block specified sites for one user group and other sites for another user group but it looks like there is only one url filter list.
You can use one web filter list per policy. The policy conditions are checked for each packet and if they are all true the policy action is carried out. There's no further if this than that inside a single policy.
You need to set up two policy - either one complete policy for each user group or a denying policy before the more general, permitting policy.
Do you have the "Multiple Security Policies" enabled? You'll need that feature in order to define a separate "Web Filter" policy with a different URL list. Enable it in System -> Feature Visibility -> Multiple Security Policies. After you do that, then you will have a drop-down list when you visit the page for the "Web Filter" policies and can create new ones.
Once you have more than one, you can select which one to use on each IPv4 Policy.