Two separate teams, huh....
Lemme think of that...
Ok! I got it!
Since M-Altaria and M-Salamance are going to lead, in two different teams, I advise you to be extremely careful of that big, FAT ice weakness.
You can counter those weaknesses by Pokèmon and by moves as well.
For M-Salamance, try using/putting either flamethrower or fire fang. That way, Salamance doesn't have to worry about that weakness, because 1) it is faster than all of them and 2) I'm sure he's bulky to take a hit out of it too.
But if you don't want to but flamethrower/fire fang on him, why not use Arcanine as a Pokèmon to help counter that ice weakness, and put dig on Arcanine to also get rid of the rock weakness.
However, if you wanted a full team of 4, I would suggest putting a Storm Drain Cradily on there, to help with Arcanine's weakness of water and ground. Another good Pokèmon would be Gardevior, although you don't need another mega to fit in, I thought it would be a good idea since you need a special attacker (if your going with physical Cradily, M-Salamance, and Arcanine). Or if you want, use a Gallade.
For M-Altaria, which I think would be better off as a special attacker, why not put a Gallade on here as well. I know you can get it's mega, but it's good off as it's own, too. Gallade covers that Ice and Rock weakness, allowing you to set up and sweep with both.
If you wanted a full team of 4, go for Escavalier (your wall and attacker) and Frosslass (your set uper or your special attacker). However, I can see there's a huge Fire weakness, so try putting Drill Run on Escavalier and putting flamethrower on Frosslass.
Hoped I help, and good luck on the Multi and Double battles!
DISCLAIMER: I treat Muti battles as double battles, but I'm not familiar to Multi battles either, but I tried and guess, so I may not be right.