The cyclos in Hanoi are wider than the HCMC variety, making it possible for two people to fit in one and share the fare. One common cyclo driver's ploy when carrying two passengers is to agree on a price, and then double it upon arrival gesturing 'no, no, no...that was per person'.
Aim to pay around 3USD per person for a journey in the city centre. Longer rides or night rides are double that or more. Try to negotiate in dong or dollars. You'll also find that a little bit of Vietnamese goes a long way when talking about prices.
The cyclo drivers in Hanoi are even less likely to speak English than in HCMC, so take a map of the city with you. A notebook and pencil to write your destination and to negotiate prices is useful. It's a thankless job, so don't agonise too much about giving a cyclo driver that extra few thousand dong.