It's been a long time since I ran a game and I have never run any o fthe 40k games before.

What I would like to do, first, is run a simple combat, not even a scenario or adventure; just a combat to get a headstart on the rules for both myself and the two people playing.

They have created a psyker and an assassin, which I am quite happy with. But my concerns are how to create a decent and suitable opposition for them. The last thing I want is to overwhelm them (and myself) before we begin. They are rank 1 characters, at the bottom of the ladder.

The easiest way is probably to do it in the terms of waves. Start with a few weak cultist and see how that goes, as it goes on you can bring in larger and more powerful enemies or more weakner ones. That way you can also probably get a feel for the characters and the enemies and how they measure to to one another without accidentally dropping a ton of bricks on your players.

I'm thinking of setting it during an opera where the soprano, during her aria, is going to be unwittingly used to channel Chaos.

during an opera where the soprano, during her aria, is going to be unwittingly used to...........

Awesome, consider this plot added to my GM big-bag-of-plots.

Doubly stolen plot device. Brilliant!

Also, try some stealth options too, providing small groups in a series of encounters. Making it so that failed stealth results in the more interesting encounters, but not necessarily breaking the need for stealth through the next set of challenges. That way, they remain able to continue to skirt danger or engage it when they feel confident. The first time they fail a stealth check, don't increase the number of enemies more than by 1, and increase it again the next time to 2, they will get the hint of increasing danger without actually having to be overun in the end or even having to fight those enemies.

In the Call of Cthulhu rulebook there's an opera in which Call Azathoth is cast in the third act if you perform it.