As Allen & Greenough 135e explains,
When the denominator is one greater than the numerator, only the
numerator is given.
duae partēs two-thirds
trēs partēs three-fourths, etc.
So, duas partes means two-thirds.
A bit modified because the verb was weird:
Concessi etiam eis has decimas, scilicet duas partes decimarum in molendinis et pannagiis et in ceteris dominiorum de Stotfolde [...]
And I granted to them these tithes, namely two parts of tithes as mill-houses and as privileges of feeding swine in the woods and as others of ownership of Stotfolde .....
I believe this may be from Aristotle's Posterior Analytics. In chapter 2, he wrote:
Ἀληθῆ μὲν οὖν δεῖ εἶναι, ὅτι οὐκ ἔστι τὸ μὴ ὂν ἐπίστασθαι, οἷον ὅτι ἡ
This has been translated into Latin in a way very similar to what you quoted:
Vera ergo oportet esse, quonian non ens scire non possumus; ut, diametrum esse costae commensurabilem.