One roll of Prima Duo is easily big enough to get a decent track on, cutting it into 3 or 4 pieces would give you a better area work with, depending on the room you have. Just cutting it in half makes a very long and narrow area.
To join the carpet gaffer tape does the best job, but it takes ages to put down and take up and costs a fortune. We use a decent quality sticky 2 inch parcel tape bought by the boxful from a local supplier. We did try cheap tape bought from ebay, but that ended up with cars regularly under the carpet - a 1/12th body seems to be designed to lift the tape if the car is set too low and the body scrapes the track. To lay the tape we just stretch out the tape the full length of the join, place it on the carpet while keeping it under tension then walk along it to press it down, takes very little time once you get the hang of it.
Interestingly the shiny, low grip strips of tape across the track doesn't seem to make too much difference to the cars handling, it doesn't take too long to figure out that if there is tape at the apex of a corner you wait until the car is past it before accelerating. Just make sure you don't position the track layout so a join runs down the length of a straight.
The reason the top boys use a different set of tyres for each run is consistency. When you first treat your tyres they are dry and the additive soaks in. After you have wiped it off and raced the tyres have not dried out and still contain some of the additive and some of the tyre is still softened. You treat them for the second time and it just softens the tyre more, so they generate more grip the second time you run them. Using each set of tyres for one run only makes sure you know exactly how much grip the car will have every time you put it on the track, as the grip is the same with the foams as they wear down. It also means all the tyres are exactly the same diameter at the start of every race so your gearing will be exactly the same each time. As the tyres wear and the diameter get smaller the rollout changes and you are supposed to change your gearing to compensate for this, although you're taking it a bit too seriously if you are checking rollout tables between races at a club meet.
As they aren't softened twice or more each week, using 5 sets of tyres for 5 races each week will actually last more than 5 times as long as just using one set of tyres so it is actually a little cheaper in the long run.
Personally as my club doesn't take it's racing seriously we generally just use one set of tyres continually until they are worn out as we can't be bothered spending all the time between races working on our cars and generally accept the lower grip in the first race each week. The way round the different grip levels is to put lots of additive on at first, then use less additive (on for less time) for the rest of the night, gets easier to work out how much to use when you have had a few weeks running.
Regarding additive which one you use isn't too important but you will find it is best to use the same additive as everyone else, the additive ends up in the carpet as you race and some additives just don't work well with each other.
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