what is it called when one labor union strikes in solidarity with another in another place? does it have a name?

Secondary Picketing. (That’s the British name anyway).

Famously, Margaret Thatcher made it illegal.

progress

Sympathy Strike.

Some courts have held that a sympathy strike or walkout is not in violation of a no strike clause in an employment contract. Honoring a picket line is the most common form of sympathy strike. Workers who honor legal primary pickets at their place of employment may be replaced but not disciplined. A worker who honors a primary, economic picket may be permanently replaced.

There is considerable case law on the topic.