A triad pair can consist of any two triads of the same or different qualities, as long as there are no duplicate tones among them. This is most easily accomplished by taking adjacent triads from the same scale system.
In most cases, the first pair an improviser might come across contains two Major triads a whole step apart, which is derived from the 4th & 5th degrees of the Major Scale (F & G Maj triads in C Major). Alternating these 2 triads through their inversions creates, on its own, a decidedly Mixolydian flavor.
The subject of this post, which takes it a step further - adding harmonic and melodic tension - is the triad pair based on the 3rd & 4th scale steps of the Melodic Minor scale (Eb+ & F Maj in C Melodic Minor ).