Whitefish often school up, at all times of the year. You tend to catch many, in the exact same spot, regardless of whether they are spawning.
Whitefish spawning occurs in a very specific temperature range and the time of year can vary significantly, on different rivers or different parts of the same river. If you're catching trout below whitefish, there is a very good chance the whitefish are spawning and the trout are eating the eggs. You might like to try a small (~5mm diameter), yellow, egg.
When euro nymphing, catching a lot of whitefish can also indicate that you are fishing too deep for the trout. Whitefish sit right on the bottom, looking down and sucking nymphs off the rocks. They do rise, sometimes, but the bulk of their feeding occurs at eye level or below. Trout are the complete opposite; they eat almost exclusively at eye level or above. For this reason, if you are dredging the bottom and only catching whitefish, there is a good chance your fly is too low for the trout. In my experience, during the warmer months, that is almost always the case. The only exception is winter, when the trout can be glued to the bottom and feed at the same depth as the whitefish.
Regarding fishing with emerging insects around, I rarely resort to swinging a fly. It is a fun and relaxing way to fish, but it is far less surgical than tight line or suspension nymphing. If I'm targeting the emergence, I just set up a dropper with the appropriate depth and fly. The only time I swing flies is if I need a fly-first presentation, in a slow and clear pool.