Thanks for your answers. I checked the tire pressures on a sidewall, it is 29-58 psi. I've got 40 psi from the sram tire pressure calculator, while using a little bit higher team weight to take into account the different weight distribution. So it seems to be a bad idea on tandem.
The idea with tubeless mtb tires was to get more comfort by the offroad touring. If we use max tire pressure, then probably there is no much difference between the 47 mm Marathons inflated to 70 psi and the 52 mm Mezcals inflated to 58 psi (the latter has also 2.54 smaller diameter)
The rims I use with MTB tires are DT Swiss Hybrid X491. 25 mm inner width. With Marathons I used Mavick A719, 19 mm inner width.