If possible you want to find tandem-rated forks. Anything less than this, and you run the risk of it failing when you least expect it.
I would be leery of MTB carbon forks as they would be A) pricey, and B) questionable for a tandem unless the manufacturer has tandem-rated them.

If you can find an actual T900 fork would be best, since it would have the correct steerer length/axle-to-crown height/rake/offset and would match your bike.
If you can't get your hands on one, then try to at least get the specs off of it so you could try to get something to match. Maybe one of the nice folks on the forum could measure theirs for you.

Then start looking for folks who might be selling 26"" tandem forks, perhaps one of the older Cannondale, Raleigh or Burley, etc.
You could also look at local ads for folks selling tandems who have converted theirs to front suspension. They may still have the original fork sitting in their garage.
Or you could always post a "want to buy" ad yourself and see who replies.

If after exhausting the above, then you might look into "dirt jumper" (DJ) forks. Surly used to make one called the "Instigator" which was tandem-rated. Very hard to find now. You can read about it here: https://www.zioncyclery.com/product/...k-157930-1.htm
You might find other Surly forks which look equally beefy to do the task. Otherwise there appear to be a few other brands of DJ forks which appear to be quite strong. Some examples are: Gusset DJ26, Identiti Rebate, etc.

Good luck!