Just to add a little to the above, the normal balance philosophy is to add weight to the crank to balance out 100% of the "Rotating" mass, which is basically the crankpin and the weight of the big end of the con-rod, and also 50% of the "Reciprocating" mass, which is the piston asssembly (rings+gudgeon pin) and the weight of the small (or little) end of the con-rod.
This 100%+50% applies more or less for each cylinder in most engines, however what you end up with in a single cylinder engine is the 50% reciprocating mass being "out of balance" at mid stroke, i.e. you get a fore and aft force shaking the engine. At TDC and BDC it is half way cancelling the effect of the piston etc, so you also get some up and down forces. 50% represents a good compromise between fore/aft and up/down.
However if you then add another cylinder at 90deg, what is fore/aft for one becomes up/down for the other, so the balacing reciprocating mass ends up working on both cylinders more or less ideally, so it is close to being properly balanced.
Strictly it's never quite right because the piston accelerations at TDC and BDC are different due to the con-rod rotating in opposite directions relative to the crank (think about it), and if the con-rods are side-by-side (usually) instead of forked (uncommon nowadays) then there is also an offset between the sets of forces, hence a small rotating couple. Opposed twins (BMW etc) suffer this offset effect significantly because there's also a centre web between the crank throws so that they can both be at TDC together.
The Honda NTV series (Bros/Deauville etc) have a split or staggered crankpin with a thin centre web, the cylinders are at 52deg but the firing intervals are
Front-232deg-Rear-488deg-Front, which I make to be a 76deg offset. I don't pretend to understand the exact philosophy behind this, I'll just trust Honda-san to know what he was doing.
