I am very sorry, but it seems I made a major foul-up this time.
One of the latest patches I made breaks XBoard very badly for playing
engine-engine games in a variant. The test I added to check if the second
engine supports the variant is made before that engine is actually started
(and has told XBoard what variants it plays). So the test always fails,
and XBoard refuses to switch to two-machines mode.
There is a work-around, which is to make sure the second engine has
already been started, e.g. by opening Engine #2 Settings, or playing a game
in variant normal, before using TwoMachines mode. But this is not possible
in engine matches or tournaments. I must have done one of these things
when testing the feature (probably first playing a normal game, to make sure
that was not blocked).
So I am afraid that we have to do an immediate 4.6.2 release to correct this.