It is worth noting that each player can play as a legendary hero, allowing you to have different hero combinations than you could have in single player.
A good strategy for co-op would be to have somebody play as HB and another as a legendary hero. This works out very well together because upgrading your food stash is very cheap and while HB is out supporting his tier 5 troops, your legendary hero can sit back at the base and defend, or go out to help the fighting. It is recommended with this strategy to have 3 maxed out bug banners, a level 5 food stash, and a level 5 shielder. With this you will be virtually unkillable.
A somewhat low stategy, it involves using a powerful legendary hero and simply bulldozing into their base at the immediate start of the game. Often, the enemy might be caught unaware and their food stash can be quickly destroyed before they can kill you. Beware of enemies who might use this as you might want to build some defences for your base immediately.
Pillbug Support This strategy revolves around using Pillbug to hold down bases to increase gold earned per second as well as to prevent opponents from advancing down a lane. The instant the game starts, double tap to send Pillbug to capture a far away base, after which, direct him back to assist your other hero. Try to send Pillbug near a tower or behind cover at all times. Notice where the opponent heroes are focusing their attack on and make sure Pillbug is at a suitable location that can prevent them from advancing. Your other hero should not engage in a 2v1 unless it is a tank or has a strong one-hit ability like Army Ant's Honor Smash or Aphid's Chi Slash.
Equipment: Coolant(Top priority), Armour piercing upgrade(optional depending on the opponent's heroes) Weapon upgrades(Second priority)