If the dancers making the Arch are stationary, as in Cumberland Reel, they Face each other and Take Both hands; if they are moving towards the dancer(s) who is/are about to go under their Arch, as in Postie's Jig, they Take Nearer hands.
For example, here are some Scottish Country Dances in which the term, Arch Hold, is used in either the MiniCrib or the MaxiCrib Dance instructions or both -
Isle Of Skye
Rory O' More
Shepherd's Crook (RSCDS Book 10)
Twa Rogueish E'en