Disease Control

A long time ago, a B creature went to the A universe, but he carried a disease with him they are asymptomatic carriers, millions of persons died by the plague. Since then, no people from the B universe is allowed to travel to A universe.

People from A who travel to B universe are allowed to travel back after spending time on quarantine. If they don't die, it means they didn't get sick by the plague.

Obviously, this is just a legal restriction if you really want you could travel illegally, or make a vaccine.

Energy Levels.

The universe A is at a higher level of energy compared to universe B.

Travel from A to B is free, since you already have the energy. Instead, travel from B to A requires an extraordinary amount of energy/magic.

When a traveller from A universe who is in B universe and want to come back to its original universe (A) also travel free, since he already has in himself the necessary level of energy. Maybe, when an A person travel to B, all the energy released due the lowering of its energy state is stored, so when they want to come back to its home they just must spend that already store energy.

If a B habitant wants to go to A he will need to spend huge amounts of energy/magic.