Q1. If you have a modern off-peak supply and meter, the whole house gets switched to off-peak rate at the set times. So you only need a single feed to an immersion programmer, and it switches the lower immersion at the appropriate times. A boost button powers the upper immersion.
If you have an 'old fashioned' Economy 7 seperate meter, and separate consumer unit for off-peak, you need one cable from that to the lower immersion, and another from the normal consumer unit for the boost option (and to run a programmer if needed)
Q2. I have this attangement you are asking about. A stand alone boost timer is connected to the upper immersion (hardly ever used), and a Timeguard wifi switch is connected to the lower immersion, set for a couple of hours in the morning, and again in the early evening. I can turn the HW on/off remotely if I need (which is used more than the boost timer!). Alternatively any immersion timer could be used.