There are a bunch of video DJ suites out there that can do way more than what you need but they all cost about $300. There is an open source software called Sno

There are a bunch of video DJ suites out there that can do way more than what you need but they all cost about $300. There is an open source software called Snowmix that might do what you want. Interestingly, the project is run by an amateur manned space program.