under game options ( where you can also modify game speed, fleet sizes and pirate settings) you get to chose between locked and unlocked teams. if you pick the former, alliances as set at the start of the game will last until the end with no way of changing them in between. it seems to me that this is what you are looking for.

in a 2 v 2 map your either rly confident or plain stupid to ignore an alliance offer vs an already teamed up oponent...

some good hints on making an alliance with someone, and how to spot basic backstabber behaviour

first of all, dont just ally with anyone, keep a close eye on whom u want allies with.. is he close, does he expand fast?

secondly watch his behaviour, does he swear to some people?  is he being annoyin.. using.... dumb language? most of those are annoying to have as allies, and most even demand territory (rambo personality i like to call those....)

how to keep your ally.... as soon as you have a full alliance (with vision) start talking about expanding strategy and who will secure wich possible chokepoints. also share recources/planets  if you have 2 deserts and lack crystal try trading.

this is also the first way to see if you have a possible backstabber... for example, if he shows little interest in plans he obviously already has his own, at that point you'll have to choose... stick with him and control both your enemy and impress your ally with a large fleet / economy (preferably fleet)  or unally him... or in a more stylish case... unally him when u do not need him anymore....

second and best way to see a backstabber coming, watch the story from his perspective.... what wil he gain?

(first mistake u can make here is looking into his perspective from your side...., if u turn around the perspective.. turn everything around, remove information he doesnt know!!!!

i know u didnt ask for this but just to clear up the how to ally vieuw