Yes, the topic is on-topic here. You will have to add a lot more info, though!
The full picture
Yep, asking about how to build something that combines electrical signals is on-topic.
Problems with Lack of Information
You will need to give a much more in-depth characteristic of your signals – after all, this is analog video, and that doesn't say anything about the number of rows you send, or how wide they; whether both signal sources use the same resolution, row and frame rate, length of sync periods… There's a lot that you leave undefined, and the first dozen comments will be trying to clarify all these points (if you're not getting closed lacking necessary detail), or you find someone with much time and they write a lengthy answer on what you need to recover from both signals, and how 1990's video signal processors worked, and why they had to be so complex in the absence of upfront knowledge about the signal.
Though I do recommend making very clear what the technical question is: Because what you're asking here ("I want to combine by simply merging") is plain impossible (as pointed out in the comments over at DSP.SE), because as you rightfully pointed out, you would first have to match timing.
So, if you mean to ask how to build such a complete signal combiner, you need to ask how to do that, and list your considerations so far¹ for the whole system are.
If you are really just asking about how to sum two signals, which you already assume to be synchronous, giving you a useful output signal, then do please say that².
It usually is much easier for experts in the field to answer a specific question and then to generalize from that answer for a beginner, than the other way around. So, even a small sentence like "I want to combine the video from the VGA port of my Segantendo XGameStation with the output from the SCART port of my PAL VCR; the output should go to the VGA capture card in my PC" will help them a lot giving you a better starting point.
¹ you mention aligning vsync, but that's not sufficient; also you assumed in the question over at DSP.SE that you could modify one signal source to match the timing of the other, and while I doubt you can, that'd be different for a signal combiner, as that needs to just take the signals you have
² and please make sure you mention what the desired output signal properties are. Bonus points, probably, if the readers of the question can understand how you keep the frames and lines in sync, because, hinthint, that's going to be the hard part and everyone is going to ask you about that, not to waste your time describing a solution to you that won't work in practice at all.