Actually, what's happening here is a bit different than what c.p. says.
There are two things that must be considered.
- How does one count infinitives?
- Given 1., which verb form should be used?
The question in 1. is actually something rarely brought up in Spanish classes and not commonly recognized even by natives. All nouns in Spanish are either masculine or femenine, and these two genders can also exist as plural or singular. But, everything else (nominal relative clauses and infinitives) are neuter, and neuter only exists in the singular. What this means is that no matter how many neuter items you build up in a sequence, they are still treated as if singular (exception: if they the neuter things are being contrasted or differentiated, then they are treated as plural)
So, we know that estudiar y trabajar ought to be treated as singular and arrive at 2., that is, which verb form should we use? When using copulas or quasicopulas in Spanish, occasionally you'll end up with the subject and the predicative nominative disagreeing in either number or person. When this happens with persons (el profesor ¿es/soy? yo), if a pronoun is used, the verb necessarily agrees with the pronoun, which generally means the 1st or 2nd person. If, however, the number is different and there are no personal pronouns (as in the case you cite, or, for example, La primera causa [de algo] ¿es/son? problemas [de algún tipo]), the verb will go in the plural, with some exception (a common exception being if the singular item is an uncountable noun, or if the plural item is a noun that doesn't exist in the singular).
So, going back to the sentence
Estudiar y trabajar al mismo tiempo siempre resulta un gran desafío
We know that this must be singular because estudiar y trabajar collectively are singular and so verbs would agree in the singular with them. But resultar is a quasi-copula in Spanish and so we need to look at the predicate nominative which is also singular, so there is no question — the verb must be singular.
When we go to the sentence that c.p. provides:
Estudiar y trabajar son cosas que la generación actual no quiere hacer
You'll notice that now we have the singular subject estudiar y trabajar, but with a plural predicate nominative cosas. As we've noted, when there's a number disconnect, the norm is to agree with the plural (cosas).
You might be wanting some evidence of this. To begin with, the ASALE/RAE has a description of it in their entry for ser (2.1.1.b-c) and on concordancia (4.1.c). But here's a simple way:
- Estudiar es fácil (bien)
- Trabajar es fácil (bien)
- Estudiar y trabajar es algo fácil (bien)
- Estudiar y trabajar es fácil (bien)
- Estudiar y trabajar son cosas fáciles (bien)
- ?*Estudiar y trabajar son fáciles (mal)
- Estudiar y trabajar son iguales de fácil (bien)
If you ask the average Spanish speaker, the next-to-the-last sentence will sound very odd, if not downright wrong. All the other sentences will sound perfect fine and unremarkable (the last one illustrates the contrasted situation allowing plural).