Since has remained unanswered for over a week I'll have a go at explaining the difference.
Although they both nominally mean 'While', 'pendant que' is more of a temporal conjunction indicating that indicates the time frame of an action during which something else happens.
Pendant que j'étais en France j'ai visité la tour Eiffel. - While I was in France I visited the Eiffel Tower.
Tandis que signifies a contrast and can often be translated as whereas.
Paul est un travailleur acharné tandis que son frère est un bon à rien. - Paul is a hardworking man while (whereas) his brother is a wastrel.
Hope this helps,
Salut Tom! That makes sense...and resound in my echo-chamber of previously known & spoken French!
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