Surface Pro to Inspiron 5477 AIO and second monitor

Hi,

First time poster here, please be kind if this is a real 'noob' question.

I have an Inspiron 5477 AIO with a second monitor connected via HDMI out, all working perfectly. I now also have a Surface Pro laptop for work, that I'd like to be able to connect to both the AIO and second monitor, to utilize the two large screens. I know, I know, a waste of the AIO as just a second screen, but needs must! I have managed to get these working separately with the Surface Pro, but I can't seem to work out how to get them both connected simultaneously? Is this possible, without having to buy an expensive docking station?

Thanks for any advice you can offer!

Richard