In FIDE Chess, the option for pawns to advance two spaces on their first move was introduced to speed up the opening of the game. In Horde Chess, Black could not use that power anywhere near the beginning, so that rule would not accomplish at all the same purpose as in FIDE Chess.

The Black position is inferior in Horde Chess, but applying the double space move in this case seems artifical. Why should the double space option only be for pawns on the second rank? The FIDE Chess setup only has one row of pawns, so perhaps its rules should not be grafted onto the very different Horde Chess.

In any case, I think a more radical change than the double move for second row pawns is needed to equalize the game. How about giving Black two kings, starting in the empty d8 and e8 squares, both of which must be captured for White win the game? Or maybe they could start in the corners? Black needs some way of protecting the pawns at the bases of his/her chains.