Well, if the markets are in a fixed location, a crude way would be to have triggers killing trade carts that get close to the forbidden markets.

They are near each other, it is to avoind 0 range trading. It would be more like changing market properties to make it non targeteable by the other players market, or something.

If you create two types of Trade Cart and Market, one of the Trade Carts goes to the original market, the other goes to a hidden market, which can not be built or appear in the editor.

[This message has been edited by dontstayafk (edited 02-14-2019 @ 11:01 PM).]

making them enemies perhaps?

From least to most effort (I think)

Option A.1) Use trade cogs for the one trade route and trade carts for the other. Alternate lanes where needed.
Option A.2) Use natural barriers like cliffs and/or gaia/other units like haystack/oldstonehead/rock to separate the trade routes

If that's not an option:

Option B) Instead of killing carts, just repeatedly task all carts in non-tradeable market area away from the market where you don't want them.

Option C) If you can manage it, have 1 AI player per player with a friendly gate to that player with access to the market. No other player can use the gate to the market, since they won't be allied. (sneaking in works, but it's at least a compact and viable solution, which I believe is what you want)

Option D) You can use a system of tribute triggers to replace market functionality. (like a lot of scenarios do to buy upgrades with kings for example)

Option E) In Trigger Studio, figure out which attributes under the 'accumulate attribute' condition would help you to create reward/punish triggers to incentivize p7 trading and discourage/penalize p-1234 trading. Also, warn your players not to both as instruction and in the infobox?

Option F) There's a way to bring back the fog of war + unexplored darkness somehow, but I forgot how to. In doing this, you could maybe hide allied markets except the p7 one, preventing players from clicking them.

Option G) Combine any of the above.