Let me explain.
Table sugar has an OG of approximately 1.046. I'd guess your candi sugar is the same? Maybe not, I know nothing about belgian sugar.
Anyway, add these points together. But treat them as .046 instead of 1.046. So, 2 * .046 for your table sugar is .092, plus your (presumed) .046 for your candi sugar is .138. So that's how many gravity points you added. But you must divide that by your gallons, which I presume is 5. So .138/5 = .0276. So add .0276 to your OG and there you go. 1.052 + .0276 = 1.0796 or 1.080.
As far as dissolving water, sugar is very soluble in water. Even more soluble if you boil it. I wouldn't be surprised if you could get this dissolved in 2 cups of water. What I do, at room temp, add enough water to the sugar so it's just over 1/2 dissolvable, then bring it to a boil while constantly stirring (to prevent carmelization). This will allow you to dissolve it further and prevents unnecessary dilution.