I have some winter/summer stuff for backpacking and have some very cheap target large air mattresses that I've used car camping. Myself and 5 year old son did a lot better on regular separate inflatable pads than on the large air mattresses - so we weren't disturbing each other while tossing and turning.
Big Agnes has a couple bags with pad sleeves? that they call either a "Padlock" or a "Bag Cinch" system where the sleeping bag basically slides over the sleeping pad itself to prevent your poor kid from sliding off all night. They have kids version and women's versions that I am aware of that I have considered for my family members - I think the Roxy Ann women's version can zip together or be pulled apart to lighten load when you don't need the lower temp rating and can be zipped together for double sleeping? These bags are clocking in around 10-12 liters in stuff sack and pretty heavy - I think like 30-40 oz - so hard to balance.
If money was no options, I'd do 2 zenbivy sleep systems and sleep like kings!
Other note, my son enjoys carrying a light pack - whether it's a very very small camelbak with water or a few liter kids style backpack - he now carries a couple pounds including his snacks, some backup clothes, water, pillows. Saves me some space.
I read the comments about enlightenment equipment, then went to the site, damn, they are too expensive. I looked for a quilt I would like, 1000$ no thanks.
Personally I have Big Agnes and they are high quality, in my experience, and cheaper. I have a -15 degree down sleeping bag from them thats going on about 12-15 years.
If my bag falls apart, I'll just stuff it and sew it back up.
We have a Big Agnes something or other 15 degree down bag we bought for around $300...10 years ago. It's great, although more like a 30 degree bag. Size / weight is about equal to two sleeping bags.
We also tried out Feathered Friends' Spoonbill which is well out of your price range but might be interesting to look at to see how an actually warm two person bag looks.
So the problem with any normal two-person sleeping bag is that there's an air gap between you if you don't sleep snuggled up. The Big Agnes we have has a small flap that covers that up. We have also stuffed clothes in between us when it was colder. That works for 15 to 20°, but not colder.
The reason I said the spoonbill was interesting is that it has individual draft hoods for each person so you share warmth but actually can't cuddle.
Since it's currently at full price, the Magma is about $100 more expensive than either of OP's choices. The big difference is that has 800 fill power down, where the other two are 600 and 650 fill power.
My daughter recently replaced a synthetic bag she bought while in college with an REI model, and liked their new sizing system. I think she may have bought the REI Magma. I have and like a Big Agnes 650 down bag I've had for years, but at a 15 degrees F rating, it hasn't gotten a lot of use in recent years.
I agree with u/TheBimpo that, unless they are trying to put a full set of gear into a pack of 50 liters or less, OP will do fine with either of their choices.
I have a Big Agnes single I LOVE so I’m jealous of all the folks here with the double. We have a flatlands fleece double bag and the Klymit double pad because they were the easy buttons through our wedding registry. The bag packs large for my taste but the pad is perfect. We sleep on top of each other at home and love snuggling all the same in the wild. Unless your trips are all ultralight backpacking, it’s quite cozy and romantic!
sloozeberrypie
