I play Survival from time to time. And I play alone.
This is a dilemma for me, as well. But since you and your bro play together, you have a greater chance of getting both destroyed. You just need to plan ahead; who's gonna do the juggernaut, and who's gonna do the helicopter at the moment when both comes at the same time.
However to do this successfully, you need to buy a killstreak for both situations; the plan a, or the plan b. Because juggernaut takes a long time to kill, I would start there. At the same time, the helicopter is a controlled killstreak, so you can expect automated kills half the time. Terminal's a bitch to deal with killstreaks, unless you're outside.
So, suggestion: When the helicopters come, stay in the airplane area. Killstreaks are by nature, "stupid." It can't see you when you're in places with a roof over your head. Go out, and destroy the helicopter when the killstreak stops (and it does stop), that's your chance to use the killstreak that you already bought.
Hopefully, you've distanced the juggernaut far away from your area as you destroy the helicopter.