I use a non-toxic water-based glue system in my shop – Simalfa, which unfortunately is not sold in smaller quantities than 5 gallons. The home-made solution I have used is Aleene’s Tacky Glue which is a water-based non-toxic white glue. You can mix the glue with water to make it brushable – apply glue to both surfaces of the latex and join together. Allow it to dry overnight before using it. This glue method will stiffen the foam slightly at the seam, which may be more noticeable with vertically joined pieces (joined to make a longer or wider cushion) than horizontally joined pieces (joined to make a thicker cushion). In other words, it could feel uncomfortable if it were placed in the middle of a seat. If you glue two pieces to make a thicker cushion (creating a horizontal seam) it may feel slightly firmer overall, but should perform the same as a single piece.
If you’re using a spray glue made for gluing foam together (which are usually high VOC content), the seam will be less noticeable since these glues remain more flexible when dry.