Database engine on separate OS/VM or same

   We have to deploy SQL Server 2012 as database engine to our front end application.

   We are given option to have two VMs one for SQL and one for front end.

   OR

   One VM with specs same as of two VMs combined. more CPU, Disks and RAM on same server. On two instance same specs to be divided in two VMs

   What is better to keep SQL on separate VM ? or same instance ?

   Any pros or cons or best practices on both approaches ?

Shahid Roofi