Two of a trade can never agree

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 A2V]

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]