A single part paint cures by evaporation of solvents and often also by a chemical reaction with air or water. The chemical bonds formed within the paint are not very strong, so it is relatively easy to wear and scratch it.
A two part paint cures by chemical reaction between two different chemicals which are mixed together just before painting. Once the reaction happens, it is not reversible. This means the chemical bonding in the paint is much much stronger so the paint is hard and resists scratches. The fact that the paint is a resin means it only needs a small amount of solvent to thin it, rather than quite a lot of solvent to carry it (some oil based single pack paints also do this). This also typically means it cures to a much denser film so air and water don't get through so easily, and some can also chemically react with the metal surface, blocking the surface from further chemical reactions with air and water, ie they stop it from rusting.
Epoxy and polyurethane are the two types of two part (two pack) paint you are most likely to encounter. They both work as above, but they are chemically very different and the chemical reaction that happens is not the same. Epoxies mostly cure with amines (the slightly fishy smelling part b) and polyurethanes mostly cure with isocyanates which smell like superglue.
Performance-wise, a two part paint is much better. It is harder and tougher and protects the surface better so the paint does not flake off if you get a scratch through. However, mixing tiny quantities for spot repairs is difficult (not impossible). More importantly, whilst epoxies seem to last forever in the tin, polyurethanes (topcoats) are moisture sensitive and 'go off' in the tin. That makes it very difficult to deal with scratches over the years you need the paint to last. There are some choices on compromise - you have already covered the most important protection with an epoxy primer so you could now use a single part undercoat and topcoat. This would make it easiest to touch up but it will need doing more often, or you could use an epoxy undercoat and single part topcoat (alkyd or mono-urethane) which will mean the scratches tend to get less deep and the colour part is still easy to touch up, or you could use an epoxy undercoat and a polyurethane topcoat (I would also second the Epifanes recommendation - boatpaint.co.uk had the best price last time I bought some and I have used them several times, finding them very helpful in mixing to RAL colours), using a colour scheme where the exposed areas are a different colour, so for example framing out the panels and painting the hand rails. These small areas would then be painted in a single part paint so they are easy to touch in, while the main sides which are less exposed are in the two part paint for durability.
Alec
The most uncommon chemistry, which I am currently trying out to see how well it performs in practice, is the polysiloxane cured epoxy. In theory it doesn't matt, chalk and fade like epoxy normally does. For now, single part alkyds and mono-urethanes, and two part polyurethanes appear to be the best options for topcoats.
One other thing to mention - pigment is expensive, resin is cheap. It is fairly obvious therefore how different paint brands can be sold at different prices. Paint really is one of those things where you get what you pay for, typically more pigment in the better brands. This gives better colour coverage, lower permeability to air and water and a stronger paint, particularly true in a single pack paint where the resin is not as strong. There is therefore a very good reason to use a brand based on recommendation. FWIW, International, Hempel, Jotun and Epifanes all spring to mind immediately as high quality brands. There are others too such as PPG Sigma but they are more niche, but what it does mean is if you spot something locally that appears to be a good deal, from an unknown brand, it is worth checking online whether that is an uncommon high quality brand or something cheap and cheerful.
Alec