The ruleset of the Federación Internacional de Scrabble en Español (FISE) says:
- Letras dobles
En el SCRABBLE los dígrafos RR, CH, LL aparecen en una sola ficha. Por tanto, no podrán utilizarse dos eres, ni dos eles, ni la ce y la hache para formar una doble letra.
so those rules do not allow forming the digraphs "ch", "ll", or "rr" using two tiles. The rules by Mattel, however, do not seem to forbid this, so it is not so clear. I suggest to play with the rules that make the game more fun to you.
Part of the reason for the existence of these Scrabble tiles with two letters may be that the digraphs "ch" and "ll" used to be considered part of the Spanish alphabet, but not anymore. The article does not have a date, but the removal of the status of "ch" and "ll" as letters seems to have happened between 2005 (when they were letters according to DPD) and 2010 (when the Ortografía de la Lengua Española says that they are not letters).
The combination "rr" is listed as a "digraph" (a combination of two letters with a different pronunciation than the two separate letters) in the Ortografía de la Lengua Española, but it has never been considered part of the alphabet (DPD). So the choice to make "RR" a single tile in Scrabble and not "QU" (which is also a digraph) seems to have been arbitrary.