If you want to incentivize multi-color aggro, make those decks better then there single color counterpart. The best thing I think you can do is to make sure your cube has a strong sense of color pie and avoid being overly redundant: that is, make playing one color strategically inferior to playing two colors. The main culprit here is usually red: people include tons of small drops, burn and land destruction in red, so there is no real reason to play a second color. If you reduce some of the redundancy in red's burn and aggro-friendly creatures, people will see that they need to move into a second color to get the right density of efficient creatures/removal/disruption to make a winning deck. Cross color synergy is the other way to make it work. In my cube you can see that: doublestrike red definitely needs white or green and heroic/aura red needs black or white to have enough efficient threats.
The negative approach is available too: take away single color options that make the mono-color decks non-viable, but I generally think negative approaches are less fun as they dictate choices. I'm always surprised at the decks that people come up with in my cube and a lot of that comes from just letting things fly.
Overall, I think if you pull back your single color one drops a touch (one less double per color) and then try to find to "ideal" single color aggro deck of each color, you can look at the unused cards and find how many "redundant" cards slot into that deck effortlessly and decide if some of them can be safely dropped. I mean, its not wrong if the nut single color deck can come together and 3-0, the pieces just need to be thin enough and in enough demand that it isn't the default best option because, by the nature of their limited card pool, single color decks are the least variable and arguably least interesting.