You should almost never go double harvesters. I win about 95% of the games where my opponent does that, the ones I lose are usually because I made a dumb mistake.
I'd recommend trying a deck where your goal is to win the first launch. If you can secure that first launch you now have the luxury of time to build up against your opponent with any tech units you can fit in.
For example, I tend to run rifles, missiles, and snipers for the early game. I've got hammerheads for if the enemy brings out heavy air units. Then I have wolverines and Kodiaks for my tech units to take the late game. The key unit of this deck is actually the riflemen. No matter what, you should start with one rifle and find out what your opponent started with for a unit. If it's a harvester check to see what building he builds first. If he did build a harvester, go ahead and build yours. Otherwise, you wait for him to make the first move for building a unit, then you know what exactly to build to counter him. Any kind of infantry? Build a sniper team and have them hide behind your rifles. Vehicles? Build at least two missile squads and have them team up on whatever he built. You'll melt even a buggy or rhino and come out ahead. This puts you one step ahead of your enemy. If you limit your mistakes you'll always be ahead in tiberium for the rest of the match and you've already won.
I don't know your level or what units you've been leveling, so I don't know if you can do that strategy specifically. But you can still follow the principle. Don't try to build a deck centered around some "OP combo", that won't work against a skilled opponent. The builds I run can counter any enemy unit if used correctly. Have to fight an avatar or mammoth? Kodiak don't care. Cyborgs? Send out a wolverine backed by snipers (bonus points if you have Jackson to boost the wolverine). Sandstorm? A defending Kodiak will kill it and missile squads can handle it easy. Basilisk or another Kodiak? Hammerheads.
Every person I go up against who has a "one-trick" build loses. Literally 100% of the time, no joke. At this point I'm only losing to people who have units 2-4 levels higher than mine, who are actually better than me, or when I'm testing a tweak to my build that doesn't quite work (or when the game decides to lag out...).
Gimmick builds push you towards defeat, balanced builds push you to victory. Once you have a solid build it's just up to you and your skill. You might not know every trick in the book or what to do, but keep at it and you'll get there!
