Combined sensor equipped missiles first may sound logical, as there are both weak points of each of them. But in a second thought, there are much biggest problems in fusing them, than the ones they solve. First of all, is what GGTharos mentions, the size, which degrades the range - and the device also requires power. Also, it would make the unit cost much more. However, if you have a missile, like MICA, able to equipped with both type of sensors, you can whether choose which missile to launch, while also having the option to launch them simultaneously, if you see a low probability of kill with one missile. Then you have two smaller, dependently usable ordnance, with both of them having greater range than a unified seeker equipped one.