The quality has to be there as the work was inspected by a clerk of works. Anything not up to standard (even missing pipe clips, flux marks or a squeaky floorboard) and you had to fix it in your own time for free or if someone else was sent to fix it then it was deducted from your money.
First heating install on my own was when i was 2nd year apprentice as we were really busy so i got the chance on my own. Orlit steel framed and concrete house. Back boiler (fireplace to cut out) 8 drop feed rads and sidewinder coil. Finished easily in 4 days on my own. By the next year 1 was on a system and a boiler change (bbu) in 4 days (still had collage). Before my time was out i could do 2 in a week + a bbu on collage holidays. Your best friends become a rip snorter and a kango.
When i first teamed up with another guy i found it didn't work out to plan. You think as you can do it yourself in 2 days then 2 men should be able to do it in a day. It don't quite work like that and you need to speed up more until you find the right neighbour as you can't carry passengers. I've had guys with me who i binned after 10 minutes.
As puddle said you both have to know exactly what to do without stopping to ask.
The old houses in Edinburgh (and this is now where i am most) are a pain but can still be done in a day with 2 of you easy enough if you go for it. The 1" or 7/8" floorboards take longer to lift and up here all the old houses have deafening between the boards (filled with ashes) and they can be a pain to get pipes through as the gap is sometimes less than an inch. The walls are also 27" thick solid stone, usually whin or craigleith stone so even the best core cutter takes a good half hour + constant drilling to get through.
You learn how to do things quickly without necessarily cutting corners. I must have done maybe 5000+ heating jobs so i know just about every quick (and a lot of bad) ways there is to do things and have made just about every mistake that can be made. Us older guys were doing it when heating was in its peak times and there was some serious money to be made if you were good and fast. (Next time you are crawling under floorboards just remember the guys 30 years ago who cut all the holes through the dwarf walls)
For example quick way hanging rads.
You don't need a tape. The bottom of the slot hole in the bracket is the right height for the bottom of the bracket from the top of the skirting for stelrads. (If you use different rads cut a wooden rod). Lean the rad against the wall, mark the centres of the brackets, use a bracket to mark the height each side, mark 1 side in the holes and 1 side in the slots. Drill it and screw it up and level the brackets. 3-5 mins max. You also start timing yourself on everything and trying to better it.
Respect to Puddle. 4 swaps in a day. Never did that one. 2 is the most i have did but if i had the chance i would still go for it for the holiday in Barbados