double stroller recommendation!

I have a 6 mo old and I'm 11 weeks pregnant. I need a double stroller but I'm not sure which one to get. We have the b agile britax stroller and car seat right now. Would it better to have the babies side by side? Or one in front of the other? I'm afraid of getting them inside doors by myself. Also will a brand other than britax be usable with the britax car seat? Also I do not want to spend more than like $300....so please no recommendations for the expensive $1200 strollers. Any info on double strollers would be greatly appreciated!!