I am going to try to post my questions and Kylie's answers without too much editing, except to remove things that only made sense in the context of our exchange.
Building and maintaining sexual tension while heightening the suspense of the story - How do you dovetail the two together without awkwardness (i.e. they should be focused on the serial killer after them, why are they thinking about each other)?
There is a trick to dovetailing the romantic suspense with the external suspense. In order to maintain the pace of the story, the tension from the external suspense has to continue to build. My pet peeve is when people go on a picnic, or make up an errand that takes the two of them away from the external suspense in an all too obvious ploy to bring the h/h closer together. But they sort of leave the suspense in the lurch in the meantime.
The h/h must both be vested in the outcome of the external suspense. So they are both working on the crime as law enforcement. Or maybe one is the detective and one is an investigative reporter or PI. That might bring the two of them together on the same crime, but give them opposing / conflicting goals at the beginning, which sets up conflict for the couple. Or a bodyguard story when he wants to keep her alive and she'd like to stay that way too :)
So they both have to have proximity in order for the sensual tension to grow. But because we have them working side by side, or at least coming in frequent contact before deciding to join forces, they have plenty of time for those feelings to evolve. This is great, because under these circumstances you can be working in the sexual tension AND the external tension on the same page! So maybe detective guy has come to begrudgingly respect the heroine who was foisted on him as a partner. And she's telling him her ideas or maybe what she discovered that day. And he's listening, but he's also noticing her ass when she bends down to get something out of the drawer. Or finds himself wondering what her hair looks all down around her shoulders, or better yet, across his bare chest.
Then WHOA...where did that come from? He yanks his focus back to her words, which were winding down. She looks at him expectantly. "So what do you think?" He figured it was wiser not to say what he was thinking at that precise moment. And wiser yet to keep his mind where it belonged...on catching this perp.
Okay that's just an example. But in your scene it would take about a paragraph. And if every scene has one or two mental meanderings one or the other of the h/h, you start to build that sexual tension and an expectation on the part of the reader that these two will end up together.
Now when they're past the thinking of each other stage, you move into physical contact. And with every contact, you establish more sexual tension. When it's the first kiss both of them avow it isn't going to happen again...it'll divert them from their goal. But they continue to get closer. The feelings are already there and they aren't going away. They're together much of the time. The feelings grow. When they first make love, the sexual tension builds one or both of them doesn't want more...or knows the other doesn't want strings...or one doesn't live there so there's no future...so now you have the HEA in doubt, because you have the characters (or one of them) doubting it, as well.
With every contact, you have reaction from one or both characters, as they respond to the change. That response heightens tension, as well.