Yeah I don't think I can be doing with a double! But I guess it depends on the baby...you can't always bank on a baby liking a sling, annoyingly.
I have a Beco Gemini now which is virtually the same as an Ergo in terms of design and longevity. But I started with several fabric stretchy wraps when he was a tiny baby because I liked I could cradle hold him.
I would strongly NOT recommend a Baby Bjorn - they have no lumber support and they do not support the baby's hips/pelvis in the correct position. This site gives good info on the importance of the frog leg position over dangling legs - which is what you get with the Baby Bjorn: onyababy.com/2012/09/babywearing-baby-positioning/
These are the slings I have:
For newborns:
The Close Baby Carrier - this is the one I'll probably use again the most as it is sort of half pre-wrapped so easy to put on, then you can adjust to different holds: www.babycarrier.org.uk/the-close-baby-carrier/
The Kari Me - it's lovely but some of the wraps are bit hard to do: www.babycarrier.org.uk/the-kari-me-sling/
For when he got to around six months - Beco Gemini, front, back, side, forward facing: www.becobabycarrier.com/cat-13/gemini
But there'll be loads of sling libraries around. Go and try them all out for free to see what suits you best :)