Video Joiner-Join 2 video signals in one

[Loirinho]

I have a computer with 2 independent vga outputs, where are connected 2 monitors with different images.
Comp. C - Monitor X with image X
- Monitor Y with image y

These images are complementar.

I would like to install a slave monitor with a coppy of X+Y in only one monitor.

Do you know how can i do that? what can I use?

Thanks.