Two instance on one server vs 2 server

I've one server with a database who stress the server and is blocking other db. Developer has optimized the query, but there we need more power. The SQL server is a VM in a vCloud Director tenant, we think two solution: * **Two Instance on same server**: we add a second instance on same VM. We double the system resource, in second instance we move only the stressed DB, limit CPU e RAM usage, so when the DB is in stress, the first instance have dedicated resource and there's no blocking query, when the second instance is down, the first instance has more resource... as another vantage I dont have to pay another SQL server license... * **Two server**: New dedicated server dedicated to stressed DB... as disadvantage I have to pay another SQL license and the resource are only dedicated to the single DB even when is not under stress... At this time we think the two instance solution is the right way, we are wrong? There are disadvantage that we have not consider? What you think? Thanks in advance for response... Stefano