I wonder if they could carry two or even 3 at a time if they parked the Zubr perpendicular to the carrier.
Such a carrying arrangement would also make it easier to disembark the Zubrs, although there would have to be a bit of overhang, which may be a problem.
I believe they could carry two if they were loaded at a slight angle, and they would not risk a potential stability situation.
I do not believe it would be advisable to load them completely cross-wise over the deck. Any high seas or difficult sea states would cause a dangerous stability situation for the ship, and could also damage the vessels.
Here's an example of a semi-submersible carrying four US Navy anti-mining vessels:
US Navy Military sea-Lift Command (MSC) chartered heavy-lift float-on/float-off ship MV Tern to deliver four U.S. Navy mine countermeasure ships from Long Beach, Calif., to Bahrain. After months of preparations, the four ships were safely floated off of Tern and towed into the port of Mina Salman, Bahrain.