I am unsure about why the end products of the reaction are what they are. Let's say I was combining glucose and fructose in a dehydration synthesis reaction to create sucrose. The equation is:
$$\ce{C_6H_{12}O_6 + C_{6}H_{12}O_{6} \rightarrow C_{12}H_{22}O_{11} + H_{2}O}$$
How do I know it is not:
$$\ce{C_6H_{12}O_6 + C_{6}H_{12}O_{6} \rightarrow C_{12}H_{24}O_{12}}$$
I am unsure about why the end products of the reaction [of glucose and fructose] are what they are.
Glucose and fructose could react to form a nearly infinite number of products. The identity of the product(s) depends on the conditions (temperature, pressure, solvents, catalysts, enzymes, etc.) used. Glucose and fructose could react to form sucrose through a condensation reaction, as you note. However, they could also react to form trehalulose, turanose, leucrose, palatinose, or maltulose. And that's just considering a few alpha-linked glucosyl fructosides. All of them have the formula $\ce{C12H22O11}$. But so do sugars such as kojibiose, made from two glucose moieties. Those are also potential products, under certain reaction conditions (e.g. using a kojibiose-specific transglucosylase and an glucose/fructose isomerase). And as I mentioned in a comment, there's no reason that reactions between glucose and fructose are limited to single condensations. Many products -- double dehydrations to form difructose anhydrides, or even more severe dehydrations to form hydroxymethylfurfural are possible.
Let's say I was combining glucose and fructose in a dehydration synthesis reaction to create sucrose.
Well, now you have specified a single product out of the infinity of products that are possible. If you've specified sucrose as the product, not only do you know the product has a formula of $\ce{C12H22O11}$, you also know that it is α-glucopyranosyl-(1→2)-β-fructofuranoside. Because that's what sucrose is. No other product is "sucrose".
The equation is:
$$\ce{C_6H_{12}O_6 + C_{6}H_{12}O_{6} \rightarrow C_{12}H_{22}O_{11} + H_{2}O}$$
Yes, it must be this, if you are making sucrose.
How do I know it is not:
$$\ce{C_6H_{12}O_6 + C_{6}H_{12}O_{6} \rightarrow C_{12}H_{24}O_{12}}$$
Well, it might be that $\ce{C12H24O12}$ is a product that could be formed from glucose and fructose under some bizarre reaction conditions. But even if it were, it wouldn't be sucrose. Because by definition, sucrose has the formula $\ce{C12H22O11}$.
So, by specifying that the product is sucrose, the only possible reaction is
$$\ce{C_6H_{12}O_6 + C_{6}H_{12}O_{6} \rightarrow C_{12}H_{22}O_{11} + H_{2}O}$$
There are three possibilities. The first is knowing the reaction. I.e., you just know because you rote memorised it, that one molecule of water will be lost and then you apply that.
The second is knowing the mechanism. If you take a good look at it, a water molecule must be displaced for the mechanism to work out. Anything else would generate something highly unstable.
And finally, if you know the product structure, you can calculate double bond equivalents. Both reactants have a single double bond equivalent: $(2 \times 6 - 12)/2 + 1 = 1$. This corresponds to the ring in the reactants’ structures. The product has two rings, thus it requires two double bond equivalents. The number of carbons must stay the same, though. Thus: $(2 \times 12 - n)/2 + 1 = 2$; rearranging gives $n = 22$. Two hydrogens must be ejected and that biochemically typically happens with water.
