Shall fall at once - Shall fall without resource, altogether.
In his ways - Rather “in his double ways” (as in Proverbs 28:6). The evil of vacillation rather than that of craft, the want of the one guiding principle of right, is contrasted with the straightforwardness of the man that “walketh uprightly.”
Shall fall at once - Better, shall fall in one of them (his ways). The attempt to combine incompatibilities is sure to fail. Men cannot serve God and Mammon.