You can chain one of the 2 Spell or Trap Cards on the field to the activation of “Xing Zhen Hu”. However, the remaining one will not be able to be activated after “Xing Zhen Hu” resolves.
If neither of the targeted Spell or Trap Cards is on the field when “Xing Zhen Hu” resolves, then “Xing Zhen Hu” remains on the field meaninglessly.