I want to add that I think limiting the "Pet" type of abilities to creatures you control is the right choice. Even if they allowed you to use those abilities on your team mates creatures but didn't allow the same ability to be added more than once, there would be a stacking issue. Could you imagine a [mwcard=MW1C20]Highland Unicorn[/mwcard] made into both a Pet and a Holy Avenger? 17 Life, 3 Armor, and a potential attack of 9 Dice with Piercing +1 for a total cost of 17 mana from one mage and 4 mana from his team mate.
I disagree and think it SHOULD be allowed, for that very reason.
I like tandem abilities that encourage particular team ups.
A Holy-Avenging-Pet-Unicorn is also the mage wars team version of putting all your eggs in one basket... now your two opponents only have to debuff or control one creature to oblivion instead of two separate threats. Two mages should be able to handle one double-pet between their combined arsenal and collective actions.
In my opinion, team games and battle royale style games are not the place to be conservative with rules.. that is the arena where things should get bonkers... the amount of control, damage and tandem effects of two mages is the whole point of such a playstyle and is strong enough to handle the effects of combined abilities.
Im all for:
-a friendly Druid's Raptor Vines getting Vet Tokens from my teammate being a Bloodwave Warlord.
-a friendly Anvil Throne Warlord to put a deflection Rune on my Forcemaster's Spiked Buckler or a mana discount rune on a friendly Warlock's Sectarus.
-my teammates temples counting towards my temple of light max
-A Necro/Priest team being able to use their respective clerics on each others Altar of Skulls/Temple of Asyra
-a friendly Warlock's Darkfenne Bats benefiting from my Wounded Prey marker
- two allied warlords having each others solider benefit from each others battle orders
-two Warlords are on the same team, that my battle orders wouldn't affect my team mates soldiers.
-a teammate letting me sacrifice one of his creatures on Sacrifical Altar.
and so on..
So, when we do the occasional team play, I prefer to disregard this ruling and let the abilities/control overlap. I think team play is just more fun with a "consensual control" concept instead and rather bland with a "friendy-but-not-controller" distinction. I havnt really run into a situation where it needed to be barred off (though I havnt done tons of team games)