Is there a talent that lets me use a two-handed basic weapon in one hand?

For melee weapons, no.

For ranged weapons, buy recoil gloves.

Would I need recoil gloves in power armor?

The weapon specifically says it's two-handed, would recoil gloves work with that?

For a ranged weapon, yes.
But be aware, the recoil glove was changed in the errata, it's now rare instead of common.

But I think common sense dictates that it could be difficult to use a 4 to 6 foot long sniper rifle onehanded with a recoil glove and still want to use it for sniping.

Actually referring to a Godwyn De'az Storm Bolter in one hand to be able to hold power sword in other.

Then yeah, recoil gloves and you're good to go. Doesn't say in the rules that power armour offers recoil glove functionality, ask your gm and He'll probably let you pay someone to install that at least.

space marines can shoot a bolter with 1 hand.

space marines wear power armor

we have therefore ruled that power armor has built in recoil gloves.

Astartes power armour specifically states it had recoil suppression built in.

I'd consider simply houseruling it, for the Angel-pattern Power Armour at least. Several of the Sisters Superior miniatures have a boltgun + melee weapon as well, and technically speaking a power armour gauntlet should be able to work by the same principle as a recoil glove.

That said, of course it could be that those characters simply accept the penalty. How much is it again, -10? You could also go for the happy medium and have it only be -5 in PA, just to voice an alternative.

This RPG's version of the Angel armour isn't very close to its original descriptions, anyways, lacking several features and being somewhat weaker in protection. This could have been simply because no suit of armour so far was described in a level of detail similar to the Marine one in DW. It's not surprising that there is a difference when you only commit two sentences to describe its features compared to half a page for the Astartes one.

On the other hand we should keep in mind that it might have also been seen as necessary to gimp this armour due to balancing concerns, given that characters now receive it on Rank 1 already. And seeing that the RAW requisitioning system is rather "simple", there'd be no reason to use a bolt pistol + melee weapon if you could just take a boltgun + melee weapon and aim just as well - on the contrary.

there's no talent but u can also give it an extra grip

Extra Grip
Also known as a pistol grip, this allows the weapon to be wielded in one hand without the usual –20 penalty. However, it becomes more difficult to aim accurately and its Range is halved.
Upgrades: Any Basic ranged weapon
and from the Errata:You may choose to hold the weapon with two hands, in which case it will have normal range.