A "sidecar arrangement" is when someone helps or supports another person or group in addition to their main task or responsibility. It's like having a backup or an extra helping hand for a specific purpose. Full definition
If you think true co-sleeping may be too unsafe for your baby, you can always consider some kind of sidecar arrangement. (easybabylife.com)
Remember that breastfeeding will be slightly less convenient with a snuggle nest or sidecar arrangement, since you'll have to lift your baby. (easybabylife.com)
If you are not comfortable having baby sleep your bed, consider the sidecar arrangement with the crib edged up against the side of your bed and the railing on the bed side taken down. (breastmilk.com)