The early Boeing 737 FCTM (1976) stated "When a positive rate of climb on altimeter and IVSI is observed, the captain will call "Gear Up". From this you can see that there was no verbal input from the first officer to actually call "positive rate". Both of course observed the positive rate but the call positive rate was considered superfluous. I go along with that.
And that's how it should be. Countless times in the simulator we see the PNF forgetting to call positive rate and invariably when this happens the PF dutifully omits to call gear up. It happens on too many occasions to ignore as just one of those things.
We also see many occasions where the PNF will inadvertently forget to call VR. Guess what? most times the PF will continue to trundle merrily down the runway waiting for the Rotate call that never comes until 14-25 knots past V2!. He was unconsciously waiting for a trigger word.
We see the example of one PF who opened up to 40% N1 with brakes released at the beginning of the take off run and trundle down the runway with 40% N1 for over 1500 feet before glancing querously at the PNF and saying accusingly "You haven't said "Stabilised" yet. This is because the parent airline required its PNF to call "Stabilised" when the N1 had reached 40% and stabilised at that figure.
Wherever you have a procedure (such as gear selection) where the one pilot is required to observe something personally then advise the other pilot he has done so before an action is called for, it is a fair bet that the action may not be initiated. The use of two men in the cockpit to take the straight forward action of raising the gear is a prime example. Rarely happens in single pilot aircraft strangely enough.
Best I have seen was during a touch and go landing in the simulator where the instructor called V1, ROTATE, when the aircraft was almost at the far end of the runway and obviously V1 had absolutely no relevance. But he had always called V1, ROTATE together as in most training seqences V1 and VR were within a couple of knots of each other. Pavlov's Dog syndrome in the aviation sense.
Then when you have a decent spread of say 12 knots between V1 and VR, we frequently see the PF start rotating at the call of V1 as he is used to the call being so close to each other. Causes a flurry of rudder activity when an engine is failed at the real V1 and the aircraft is dragged into the air 10 knots below the real VR.