Two people on the same trade, in the same field, that have the same interests
are both too envious, each imagining that the other is cleverer or better off
than he. The proverb is traced back to 1630 in written form.

> It is a common rule, and ’tis most true, Two of one trade never loue.
> [1630 Dekker _Second Part of Honest Whore_ II. 154]

> Two of a Trade can seldome agree.
> [1673 E. Ravenscroft _Careless Lovers_ A2 V]

> In every age and clime we see, Two of a trade can ne’er agree.
> [1727 Gay _Fables_ i. xxi.]

> Two of a trade, lass, never agree! Parson and Doctor!—don’t they love
rarely, Fighting the devil in other men’s fields!
> [1887 G. Meredith _Poems_ (1978) I. 148]