Three parts: two auras that point in opposite directions, and a mustering permission.
Daemonic Illusions (Aura), granted to your SCINTILLATING LEGIONS units. While a friendly THOUSAND SONS PSYKER unit is within 6 inches of and visible to this unit, models in that unit have a 4+ invulnerable save against ranged attacks.
Mortal Sorcery (Aura), granted to your THOUSAND SONS units. While a friendly SCINTILLATING LEGIONS PSYKER unit is within 6 inches of and visible to this unit, that daemon unit has the Cabal of Sorcerers ability.
Read the second one carefully, because its grammar is unusual and it is the more valuable of the two. The ability is granted to the daemon unit, not to your Thousand Sons unit. What it means in practice is that a daemon Psyker standing close to your own troops becomes another model that can attempt Rituals. Since the army rule caps you at one attempt per model and four Rituals per turn, adding eligible casters is how you actually reach that ceiling, and daemons are a cheaper way to buy caster bodies than more Sorcerers.
Both auras require 6 inches and visibility, and both are reciprocal in the sense that the two halves of your army have to stay near each other to get anything. This is not a detachment where you deploy daemons on one flank and Rubricae on the other. It is a detachment about a combined battle line.
The mustering permission. You can include SCINTILLATING LEGIONS units even though they do not have the THOUSAND SONS Faction keyword. The combined points cost of such units is capped by battle size: up to 500 points at Incursion, 1,000 at Strike Force, 1,500 at Onslaught. And no SCINTILLATING LEGIONS model can be your WARLORD.
That 1,000-point cap at Strike Force is generous enough to make this a genuinely mixed army rather than a small allied contingent, and it is worth planning against deliberately: half your list can be daemons.