In fact, that's what he's already doing... Except that it copies the sheet to have both in the same file. And I think that's why he wants the changes to happen on both at the same time.
After that, to try to find a solution, it would be necessary to answer the question of gt22.
The cartouche is integrated into the background so changing it without changing the background seems complicated to me.
It may be possible to manage this with a macro: clicking on a button allows you to generate the same shot with a different block, but it is beyond my skills and will, in any case, be developed in a specific way.
I think that the @Jose-accessa solution is the best, you always work with your cartridge and when it comes to exporting or having your client validate you change the basemap.
As S.B says, the best thing to do is to use layers.
We put one title block in one layer and the other in another layer and we just have to activate the layer and therefore the title block we want to see.
No, in fact he wants to be able to change cartridges quickly on the same plane, one cartridge for his plans, another for when he sends to his client, so with the client's cartridge, but either one or the other.
Or maybe I didn't understand anything, but I think I'm right:)
@jose-accessa: I don't agree with you about layers! even if it's not as easy as on Autocad, I regularly use layers on my plans. In this case, as I say, you just have to make 2 different cartridges on 2 different layers and display only the one you need! On the other hand, I haven't tested this principle on the backgrounds but there's no reason why it shouldn't work the same way as on the main sheet.
I did a little test, and this is the result. I just made a copy and paste of my background from one layer to another. In your application, you can make 2 different cartridges with the values (retrieved from the 3D) you want, just switch from one layer to another.
Well, I just tested the layer solution and quite frankly, I'm amazed.
I knew the function under Autocad since it is used a lot but not under SW and I think that creating a background with 2 cartridges under 2 different layers is the best solution.